Never got around to an Airfix brake van, was into N scale early on...
That said, I have a number of the later Bachy and Hornby vans and I don't fancy trying to cut a door out and reuse it. The glazing is integral forming the wall and door, suitably painted. An etch would make an open door easy, especially if it is open against wall as pictured above on John's Flickr site. You'd only have to paint the door's inside, no color matching of the exterior...
I should stay on topic.