Flat boards of fairly thin aluminum work with the Pacemaker with an accurately sized light trap attached to the back. This can be plywood, foam rubber, felt, foamcore, or almost anything else.
Great. I have lots of the "cold clips" once packaged with Polaroid cameras for developing film in cold weather. The aluminum is perfect for this, I believe, though I'll have to go into the studio now and check. The light traps are easy. I think I'll try backing the board in felt, to make the seal more even all around. If it works, I'm good to go with all those weird lenses I have/make/repurpose, etc.