I really like the idea of a plastics. Low weight would be a boon and machining would likely be easier and cheaper. I'll look into those..
This question probably deserves a huge answer, but just for the sake of asking, what kind of data would you need to design something like this? If we understood the optical needs, then I could hand-draw something encompassing the basic design once I had reasoned it out in my noggin. It'd be nice to get a more in-depth look at the inside of one of these cameras and work from that.
FYI, the reflectors are not actually glass mirrors, but are ultra-thin pellicles that have been vacuum deposited with some kind of reflective coating. National Photocolor, a company who actually manufactured these cameras back in the day, still makes such pellicles to this day. You'd need 2, and a 5x7" one in a frame is about $250. A smaller one might even be less.
I think a camera designed to take 120 film backs would be an awesome first start; though 4x5" might make more sense at first.