How to return a list of available time slots via a forms ValidationError

models.py

from django.db import models
from django.utils import timezone
from django.urls import reverse
from django.contrib.auth.models import User

class Customer(models.Model):
    username = models.ForeignKey(User,on_delete=models.CASCADE)
    name = models.CharField(max_length=20,null=True)

    def __str__(self):
        return self.name

# Create your models here.
class Booking(models.Model):
    customer_name = models.ForeignKey(Customer,on_delete=models.CASCADE,null=True)
    username = models.ForeignKey(User,on_delete=models.CASCADE)
    qty_plts = models.PositiveSmallIntegerField(default=1)
    cbm = models.PositiveSmallIntegerField(default=1)
    created_date = models.DateTimeField(default=timezone.now())
    delivery_date = models.DateField(null=True)
    delivery_time = models.TimeField(null=True)
    booking_number = models.CharField(max_length=50,unique=True)

    def __str__(self):
        return self.booking_number

    def save(self, **kwargs):
        if not self.booking_number:
            self.booking_number = f"{self.delivery_date:%Y%m%d}{self.delivery_time:%H%M}"
        super().save(**kwargs)

    def get_absolute_url(self):
        return reverse('bookmyslot:detail',kwargs={'pk':self.pk})

forms.py

from django import forms
from bookmyslot.models import Booking,Customer
from bootstrap_datepicker_plus import DatePickerInput
import datetime as dt
from django.utils import timezone


HOUR_CHOICES = [(dt.time(hour=x), '{:02d}:00'.format(x)) for x in range(7, 13)]

class BookingForm(forms.ModelForm):

    def __init__(self,*args,**kwargs):
        user = kwargs.pop('username',None)
        super(BookingForm,self).__init__(*args,**kwargs)
        self.fields['qty_plts'].label = "Quantity Of Pallets"
        self.fields['cbm'].label = "Shipment CBM"
        self.fields['delivery_date'].label = "Delivery Date"
        self.fields['delivery_time'].label = "Delivery Time"
        self.fields['customer_name'].label = "Customer Name"
        self.fields['customer_name'].queryset = Customer.objects.filter(username=user)

    def clean(self):
        cleaned_data = super(BookingForm,self).clean()
        booking_number = f"{cleaned_data.get('delivery_date'):%Y%m%d}{cleaned_data.get('delivery_time'):%H%M}"
        if Booking.objects.filter(booking_number=booking_number).exists():
            raise forms.ValidationError("Requested slot is already booked, please choose another time")

    class Meta:
        model = Booking
        fields = ('customer_name','qty_plts','cbm','delivery_date','delivery_time')
        widgets = {'delivery_date':DatePickerInput(options={"daysOfWeekDisabled":[0,6],"minDate":timezone.now().date().strftime('%Y-%m-%d')}),
                    'delivery_time':forms.Select(choices=HOUR_CHOICES)}

views.py

from django.shortcuts import render

# Create your views here.
from .models import Booking,Customer
from .forms import BookingForm
from django.urls import reverse,reverse_lazy
from django.contrib import messages
from django.contrib.auth.mixins import LoginRequiredMixin
from django.views.generic import (ListView,DetailView,CreateView,UpdateView,DeleteView,TemplateView)

class BookingCreate(LoginRequiredMixin,CreateView):
    login_url = '/login'
    redirect_field_name = 'bookmyslot/booking_detail.html'
    model = Booking
    form_class = BookingForm

    def get_form_kwargs(self, **kwargs):
        form_kwargs = super(BookingCreate,self).get_form_kwargs(**kwargs)
        form_kwargs['username'] = self.request.user
        return form_kwargs

    def form_valid(self, form):
        self.object = form.save(commit=False)
        self.object.username = self.request.user
        self.object.save()
        return super().form_valid(form)

I am trying to figure out how to return a list of available time slots, using a ValidationError. There are 6 time slots [delivery_time] to choose from on any given date -> [7,8,9,10,11,12] The booking_number field is a unique id that is a concatenation of the delivery_date and delivery_time, which is generated each time a user successfully creates a booking.

So, let's assume there are 3 existing bookings for 2021-10-21 at 7:00,08:00 & 10:00, which are saved in the Booking model with the following booking numbers: 202110210700 202110210800 202110211000

Assuming a user tries to book over a slot that already exists e.g. 202110210700, the validation error should return "Requested slot is already booked, please choose another from one of these available slots: 09:00 11:00 12:00

How can I achieve this?