One could make them from plastic or metal U-profiles. Better though seems making two U-frames from plastic sheet of the thickness of the flap. One with inner size the same as the flap and a second one with same outer size as the first frame, but smaller opening. Both are glued together. The first frame is stuck with doublesided adhesive tape to back of camera.
Sounds like it may be beyond my skills but I will keep it in mind as an option. Thank you.