Django: using mptt-django to allocate products to categories in a many-to-many relationship

I have a many-to-many relationship between products and categories. I am using mptt to manage the display of categories on the admin (which works fine) and so I can allocate products to categories.

While the categories are working fine, it's the product side I'm having problems with.

In my models.py I have (truncated for brevity):

from mptt.models import MPTTModel,TreeForeignKey,TreeManyToManyField

class ProductCategory(MPTTModel):
    parent = TreeForeignKey('self', null=True, blank=True)

class Product(models.Model)
    categories = TreeManyToManyField(ProductCategory)

In my admin.py I have

from django.forms import CheckboxSelectMultiple
from .models import Product,ProductCategory
from django.db import models
from django.contrib import admin
from mptt.admin import MPTTModelAdmin
from mptt.models import TreeManyToManyField

class ProductAdmin(admin.ModelAdmin):

   formfield_overrides = {
      TreeManyToManyField:{'widget':CheckboxSelectMultiple},
   }

This almost gets the job done but all of the categories are displayed with checkboxes which is not what I want since you don't want to add products to categories which have sub categories.

What I want is the display of categories such that you can only add products to categories which don't have children, so in this example

Category 1
Category 2 sub sub 
Category 3 sub

And the labels would be

Category 1
Category 2 > Catgory 2 sub > Category 2 sub sub
Category 3 > Category 3 sub

Many thanks for any help!

UPDATE

Hi, I realise me question may be too broad!

As a Django newbie, what I think I need to do is create a custom widget possibly overriding the CheckboxSelectMultiple widget, that I can apply to my 'categories' model attribute.

At the moment, I am using formfield_overrides for the TreeManyToManyField as discussed above

formfield_overrides = {
    TreeManyToManyField:{'widget':CheckboxSelectMultiple},
}

Not only does this not give the desired result but also rather than using the standard CheckboxSelectMultiple for all TreeManyToMany fields, can I specify this widget just for my 'categories' attribute?

Notwithstanding this aside, would it be possible to show me how I can create a custom widget, passing the information about the category tree and selected states (for a product which has already been allocated to categories)? I've been trying to sort this myself but have been struggling - any help much appreciated!

UPDATE FOLLOWING KARANTSTHR COMMENT

Here is a screenshot of the categories - all good in showing the categories with no children, but can we show the parents to these categories in the label as described above?

UPDATE 2 TO KARANTSTHR

Here is a screenshot with your implementation.