Cutom WPF WrapPanel with overridden OnVisualChildrenChanged won't work as expected

I am working on an app on Windows, and I want a Panel that layouts its children like this:

The closest I find in WPF panels is a WrapPanel with a vertical orientation which has a layout like this:

My solution so far has been creating a derived class from WrapPanel and rotating it and each of its children 180 degrees so that my goal is achieved:

public class NotificationWrapPanel : WrapPanel
{
    public NotificationWrapPanel()
    {
        this.Orientation = Orientation.Vertical;
        this.RenderTransformOrigin = new Point(.5, .5);
        this.RenderTransform = new RotateTransform(180);
    }
    protected override void OnVisualChildrenChanged(DependencyObject visualAdded, DependencyObject visualRemoved)
    {
        var addedChild = visualAdded as UIElement;
        if (addedChild != null)
        {
            addedChild.RenderTransformOrigin = new Point(.5, .5);
            addedChild.RenderTransform = new RotateTransform(180);
        }

        base.OnVisualChildrenChanged(addedChild, visualRemoved);
    }
}

The problem is that the addedChild.RenderTransform = new RotateTransform(180) part in the overridden OnVisualChildrenChanged does not seem to work. Any solutions (even very unrelated to my approach) are welcomed.

UPDATE:

I re-examined my code and realized I am changing RenderTransform somewhere else, preventing this from working. So my problem is solved. However, I'd appreciate if you offer any solutions that may be more elegant, e.g. using MeasureOverride/ArrangeOverride.