Well, no reply from Mr. Hama, but that's okay. He's probably very busy. Instead, I took the 'door' from a Tower TLR (with red window) and found that it fit 98% perfectly well onto the Ricoh. It seems light tight, if not a bit tight to latch. I've taken a few shots, we'll see how they turn out (differences in the pressure plate distance? Who knows..)
I was surprised that the film door fit like that. I was also surprised that the waistlevel finder from the ricoh fit the tower! And the focus was maintained perfectly as I checked with a piece of ground glass. The ricohmatic screen is great; a piece of glass and a fresnel. It's very bright. No accu matte, beattie or maxwell but it's sharp enough to use indoors in near-dim lighting.