DDR,
A few years back, I made a diorama of a 1995 Virginia State Police Caprice Classic, sitting in the middle of a downtown street, drivers door open, and the officer standing in the opening, talking on his radio, while looking at a drunk crawling in the street in front of him.
I removed the door from the body using a new (sharp) hobby knife, heated over a candle. The same process was used on door section of the interior tub. I used pieces of spru between the inner door and interior upholstry to get the proper spacing, then used embalmers restorative art wax to make the door end, coated with super glue and painted. I used a thick piece of spru to securly attach the door to the car, then modeled hinges to give an authentic appearance.
Do all the "finishing" work before attaching the door to prevent accidental break off (and the pulling out of your hair).
You can use the same wax/glue/paint technique to fill in the open door spaces on the car body as well. I'm sure there are other affective techniques, but this provided satisfactory results for me.
Happy modeling!
Frank