That's often my method of solving problems. If I ignore it and get on with doing something else, a solution often presents itself.
The Mamiya pressure plate mechanism is quite well designed and might be worth copying. Otherwise a rigid plate attached to the back by a piece of foam or sponge might work. You need a pair of rails for the film to run against then another pair of rails slightly higher so that when the pressure plate is up against them, there is just enough gap for the film and its backing paper.
How much space do you have between the film plane and the back?