Indeed, thanks for the link. 5x7" mirrors are $100 a piece. Doable if you could figure out the rest easily enough.
A great design would be something that uses 3 6x7cm backs so you could have roll-film capabilities, in which case you could use the 75mm˛ mirrors.
Also, I think that any design using glass mirrors, as opposed to ultra-thin pellicles, would require other clear glass in the optical path to negate ghost reflections from the light passing through 2 surfaces per mirror, but Edmund's have AR coatings that might(?) obviate this step.