Even a car as small and lite as my sports car when it was under going rehab the guy put a steel bar frame in the door spaces when he took them off. If the doors are left off for a period of time the weight of the car allows it to start bowing. Think of a board with the ends balanced on two walls. If you appply weight to the center the board starts to sag in the middle. Doing this for an extended period will cause a permanaent sag. That being said a whole door is not needed aan arm bar would suffice and be opened or closed