The 4x5 stereo pinhole camera I'm designing may get foam-core construction film holders (which would hold paper just as readily, and cost a lot less than modern double dark slides). And that, in turn, will simplify the process of converting to 5x7 single sheet; just pull out the center septum and change the tape on the pinholes; one shutter with three arms will cover all three and work from the same cable release.