Erie, can you give us more detail on your plans for film holders? I've given them some thought, and I always get back to the point where you need a felt light trap in the darkslide slot. I've made a crude filmholder out of 1/4" thick black foamcore board, but the darkslide is designed not to come out; in its extended position it serves to fill the slot as a sort of light seal....then flimholders, of some sort. Bookform probably, as they're the easiest to build with cave tools...
Obviously a better solution is needed, and one of the reasons why I went with a falling plate design for my pinhole cameras - but that way isn't as accurate, focus-wise, so it won't work well with glass-lensed cameras. I must admit that I use 4x5 film holders in my latest pinhole camera, but I have ambitions for 8x10 with improvised lenses, and I can't justify paying $70 a pop for film holders.
~Joe