Wrap it inside a FrameLayout so that there is no Size issue.

  <FrameLayout
                android:layout_width="@dimen/_20dp"
                android:layout_marginStart="@dimen/_15dp"
                android:layout_marginEnd="@dimen/_15dp"
                android:layout_height="match_parent"
                android:orientation="vertical">

                <SeekBar
                    android:layout_width="150dp"
                    android:layout_height="30dp"
                    android:layout_gravity="center"
                    android:rotation="270" />
  </FrameLayout>

I tried in many different ways, but the one which worked for me was. Use Seekbar inside FrameLayout

<FrameLayout
    android:id="@+id/VolumeLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_above="@id/MuteButton"
    android:layout_below="@id/volumeText"
    android:layout_centerInParent="true">
        <SeekBar
        android:id="@+id/volume"
        android:layout_width="500dp"
        android:layout_height="60dp"
        android:layout_gravity="center"
        android:progress="50"
        android:secondaryProgress="40"
        android:progressDrawable="@drawable/seekbar_volume"
        android:secondaryProgressTint="@color/tint_neutral"
        android:thumbTint="@color/tint_neutral"
    />

And in Code.

Setup Pre Draw callback on Seekbar, Where you can change the Width and height of the Seekbar I did this part in c#, so Code i used was

            var volumeSlider = view.FindViewById<SeekBar>(Resource.Id.home_link_volume);

            var volumeFrameLayout = view.FindViewById<FrameLayout>(Resource.Id.linkVolumeFrameLayout);

            void OnPreDrawVolume(object sender, ViewTreeObserver.PreDrawEventArgs e)
            {
                volumeSlider.ViewTreeObserver.PreDraw -= OnPreDrawVolume;
                var h = volumeFrameLayout.Height;
                volumeSlider.Rotation = 270.0f;
                volumeSlider.LayoutParameters.Width = h;
                volumeSlider.RequestLayout();
            }

            volumeSlider.ViewTreeObserver.PreDraw += OnPreDrawVolume;

Here i Add listener to PreDraw Event and when its triggered, I remove the PreDraw so that it doesnt go into Infinite loop.

So when Pre Draw gets executed, I fetch the Height of FrameLayout and assign it to Seekbar. And set the rotation of seekbar to 270. As my seekbar is inside frame Layout and its Gravity is set as Center. I dont need to worry about the Translation. As Seekbar always stay in middle of Frame Layout.

Reason i remove EventHandler is because seekbar.RequestLayout(); Will make this event to be executed again.

You can do it by yourself - it's now so difficult. Here is an example from my project: https://github.com/AlShevelev/WizardCamera

Let start from settings (attrs.xml).

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="ExpositionBar">
        <attr name="button_icon" format="reference" />
        <attr name="button_icon_size" format="dimension" />

        <attr name="stroke_width" format="dimension" />

        <attr name="stroke_color" format="color" />
        <attr name="button_color" format="color" />
        <attr name="button_color_pressed" format="color" />

        <attr name="min_value" format="float" />
        <attr name="max_value" format="float" />
    </declare-styleable>
</resources>

Here is a couple of utility functions:

fun <T: Comparable<T>>T.fitInRange(range: Range<T>): T =
    when {
        this < range.lower -> range.lower
        this > range.upper -> range.upper
        else -> this
    }

fun Float.reduceToRange(rangeFrom: Range<Float>, rangeTo: Range<Float>): Float =
    when {
        this == rangeFrom.lower -> rangeTo.lower
        this == rangeFrom.upper -> rangeTo.upper
        else -> {
            val placeInRange = (this - rangeFrom.lower) / (rangeFrom.upper - rangeFrom.lower)
            ((rangeTo.upper - rangeTo.lower) * placeInRange) + rangeTo.lower
        }
    }

And at last, but not least - a class for vertical seek bar:

class ExpositionBar
@JvmOverloads
constructor(
    context: Context,
    attrs: AttributeSet? = null,
    defStyleAttr: Int = 0
) : View(context, attrs, defStyleAttr) {

    private val drawingRect = RectF(0f, 0f, 0f, 0f)
    private val drawingPaint = Paint(Paint.ANTI_ALIAS_FLAG)

    private val strokeWidth: Float

    @ColorInt
    private val strokeColor: Int
    @ColorInt
    private val buttonFillColor: Int
    @ColorInt
    private val buttonFillColorPressed: Int

    private val icon: VectorDrawable

    private val valuesRange: Range<Float>

    private var centerX = 0f
    private var minY = 0f
    private var maxY = 0f

    private var buttonCenterY = 0f
    private var buttonRadiusExt = 0f
    private var buttonRadiusInt = 0f
    private var buttonMinY = 0f
    private var buttonMaxY = 0f
    private var buttonCenterBoundsRange = Range(0f, 0f)

    private var iconTranslationX = 0f
    private var iconTranslationY = 0f

    private var isInDragMode = false

    private var onValueChangeListener: ((Float) -> Unit)? = null

    private var oldOutputValue = Float.MIN_VALUE

    init {
        val typedArray = context.obtainStyledAttributes(attrs, R.styleable.ExpositionBar)

        icon =  typedArray.getDrawable(R.styleable.ExpositionBar_button_icon) as VectorDrawable
        val iconSize = typedArray.getDimensionPixelSize(R.styleable.ExpositionBar_button_icon_size, 0)
        icon.setBounds(0, 0, iconSize, iconSize)

        strokeWidth = typedArray.getDimensionPixelSize(R.styleable.ExpositionBar_stroke_width, 0).toFloat()
        drawingPaint.strokeWidth = strokeWidth

        strokeColor = typedArray.getColor(R.styleable.ExpositionBar_stroke_color, Color.WHITE)
        buttonFillColor = typedArray.getColor(R.styleable.ExpositionBar_button_color, Color.BLACK)
        buttonFillColorPressed = typedArray.getColor(R.styleable.ExpositionBar_button_color_pressed, Color.BLUE)

        val minValue = typedArray.getFloat(R.styleable.ExpositionBar_min_value, 0f)
        val maxValue = typedArray.getFloat(R.styleable.ExpositionBar_max_value, 0f)
        valuesRange = Range(minValue, maxValue)

        typedArray.recycle()
    }

    override fun onSizeChanged(w: Int, h: Int, oldw: Int, oldh: Int) {
        super.onSizeChanged(w, h, oldw, oldh)

        drawingRect.right = width.toFloat()
        drawingRect.bottom = height.toFloat()

        buttonCenterY = drawingRect.centerY()

        recalculateDrawingValues()
    }

    override fun onDraw(canvas: Canvas) {
        drawingPaint.color = strokeColor
        drawingPaint.style = Paint.Style.STROKE

        // Draw the center line
        canvas.drawLine(centerX, minY, centerX, buttonMinY, drawingPaint)
        canvas.drawLine(centerX, buttonMaxY, centerX, maxY, drawingPaint)

        // Draw the button
        canvas.drawCircle(centerX, buttonCenterY, buttonRadiusExt, drawingPaint)
        drawingPaint.style = Paint.Style.FILL
        drawingPaint.color = if(isInDragMode) buttonFillColorPressed else buttonFillColor
        canvas.drawCircle(centerX, buttonCenterY, buttonRadiusInt, drawingPaint)

        // Draw button icon
        canvas.translate(iconTranslationX, iconTranslationY)
        icon.draw(canvas)
        canvas.translate(-iconTranslationX, -iconTranslationY)
    }

    @SuppressLint("ClickableViewAccessibility")
    override fun onTouchEvent(event: MotionEvent): Boolean {
        if(!isEnabled) {
            return false
        }

        when(event.actionMasked) {
            MotionEvent.ACTION_DOWN -> {
                if(isButtonHit(event.y)){
                    isInDragMode = true
                    invalidate()
                }
            }
            MotionEvent.ACTION_MOVE -> {
                if(isInDragMode) {
                    buttonCenterY = event.y.fitInRange(buttonCenterBoundsRange)
                    recalculateDrawingValues()
                    invalidate()

                    val outputValue = buttonCenterY.reduceToRange(buttonCenterBoundsRange, valuesRange)
                    if (outputValue != oldOutputValue) {
                        onValueChangeListener?.invoke(outputValue)
                        oldOutputValue = outputValue
                    }
                }
            }
            MotionEvent.ACTION_UP,
            MotionEvent.ACTION_CANCEL -> {
                isInDragMode = false
                invalidate()
            }
        }
        return true
    }

    fun setOnValueChangeListener(listener: ((Float) -> Unit)?) {
        onValueChangeListener = listener
    }

    private fun recalculateDrawingValues() {
        centerX = drawingRect.left + drawingRect.width()/2
        minY = drawingRect.top
        maxY = drawingRect.bottom

        buttonRadiusExt = drawingRect.width() / 2 - strokeWidth / 2
        buttonRadiusInt = buttonRadiusExt - strokeWidth / 2
        buttonMinY = buttonCenterY - buttonRadiusExt
        buttonMaxY = buttonCenterY + buttonRadiusExt

        val buttonCenterMinY = minY + buttonRadiusExt + strokeWidth / 2
        val buttonCenterMaxY = maxY - buttonRadiusExt - strokeWidth / 2
        buttonCenterBoundsRange = Range(buttonCenterMinY, buttonCenterMaxY)

        iconTranslationX = centerX - icon.bounds.width() / 2
        iconTranslationY = buttonCenterY - icon.bounds.height() / 2
    }

    private fun isButtonHit(y: Float): Boolean {
        return y >= buttonMinY && y <= buttonMaxY
    }
}

You can use it as shown here:

<com.shevelev.wizard_camera.main_activity.view.widgets.ExpositionBar
    android:id="@+id/expositionBar"
    android:layout_width="@dimen/mainButtonSize"
    android:layout_height="300dp"
    android:layout_gravity="end|center_vertical"

    android:layout_marginEnd="@dimen/marginNormal"
    android:layout_marginBottom="26dp"

    app:button_icon = "@drawable/ic_brightness"
    app:button_icon_size = "@dimen/toolButtonIconSize"
    app:stroke_width = "@dimen/strokeWidthNormal"
    app:stroke_color = "@color/mainButtonsForeground"
    app:button_color = "@color/mainButtonsBackground"
    app:button_color_pressed = "@color/mainButtonsBackgroundPressed"
    app:min_value="-100"
    app:max_value="100"
/>

Voila!

By using RotateLayout, having vertical SeekBar is a breeze. Just wrap that horrible SeekBar into it and Bob is your uncle:

<com.github.rongi.rotate_layout.layout.RotateLayout
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:angle="-90"
    >

        <androidx.appcompat.widget.AppCompatSeekBar
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
        />

</com.github.rongi.rotate_layout.layout.RotateLayout>

https://github.com/rongi/rotate-layout

Simple answer

Instead of using android:rotation="270" inside of a seek bar, use it inside of a FrameLayout that wraps around it.

 <FrameLayout
    android:background="@color/gray"
    android:layout_width="300dp"
    android:layout_height="5dp"
    android:layout_marginEnd="-126dp"
    android:rotation="270"
    android:orientation="vertical"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintTop_toTopOf="parent">
<SeekBar
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    />
</FrameLayout>

To get my frame layout to be 24dp margin right I calculated width -150dp + 24dp because the frame layout is first drawn horizontally and then rotated vertically.