If you go with plastic, I'd recommend black ABS. It is easy to machine, tough and bondable. Delrin/acetal machines well, but it is a bit brittle (relatively) and not very bondable. UHMW is can't really be glued at all. Coefficient of expansion really isn't all that important for what you are doing as long as the material is homogeneous (wood is not). For construction, CNC is the way to roll. All this said, wouldn't it be easier to just use three filters and three sheets of film in a 4x5? I realize it isn't as cool, but it would work as long as the scene isn't moving much.
Thanks Mark; being able to use adhesives would undoubtedly be helpful, although I was imagining that UHMW-P could be bolted together if there was a good way to "sink" thread-inserts into it. But ABS sounds good.
Taking sequential exposures is fine for still lifes, but one-shots were invented to fulfill the need for people & action photography.
Though more importantly I guess, I see the act of building something, discussing some kind of optical system, or investigating any number of seemingly archaic things as an end in it's own right. I'm not necessarily striving to find practical solutions that serve photography, rather, the study is enough. If in the end these investigations can be served by photography, well that's awesome.
Make sure the glue is black to light and infrared. Hate to see a light leak from a seam.
How about making a three-lens camera that partitions one sheet of film in thirds.
Angle the three lenses so they aim at one focus point.
Then just sit back and enjoy the tricolor rainbow where parallax throws some of the image out of register.