John, precise measures would help inform possible solutions. For example, if I understand your problem, you've got something like an 8x10 camera back that's 11 1/2 inches square and a back that's 12 inches square. You don't want to permanently modify either one.
If this is the case, I would fabricate a "step-up frame" out of 1/4 x 1" basswood. Make an inside frame 11 1/2" o/s dimension to mate with the camera and attach with pins or whatever is needed to mate with hardware that is already on the camera so that the frame is removable. I would glue a 12" o/s 1" x 1/4 frame to the outside of the first frame (overlapping by 1/2 inch) and install hardware on it to mate with the back you have. No changes to either camera or back.
Finish the frame out by gluing in strips of 1/8 x 1/2" basswood for light traps, as needed on both camera and back sides. Paint the frame flat black on the inside.
Materials and tools needed: basswood strips and brass strips and screws, available at a good hardware or craft store; a mitre saw to cut the wood square on the ends; an 18 inch steel ruler; wood glue; a right angle to glue the wood frame against and hold in place until dry enough to move (just a couple of minutes). Oh, and your time, of course, which is what most of us are short of.
Bottom line: if you've got a wooden camera and a wooden back then I would make an adapter out of wood and use similar metal hardware. I wouldn't try anything as flimsy as putty or foamboard and tape. My two cents.
If this seems like a workable solution and you need help with it, let me know. I'll be glad to help out.
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