All you really need is a mirror set at 45deg. angle. I don't think it even needs to be a first surface mirror, but perhaps that would be better. But then I suppose you want the image corrected(I'm so used to TLR's and view cams). Find a dead SLR and rip the pentaprism from the finder, use 100% silicone caulk to glue it in place. Just a few small dabs will suffice.