not sure if anyone else is in this boat, but thought i would mention this.
i shoot with olde portrait lenses on a 5x7, and rather than shooting film and not having any way to "proof" i thought of some wacky way of reducing my 5x7 back to 4x5. since i use a camera with a fixed 5x7 back ( it is a szabad camera) there is no way i could use a standard reducing back, so i poked around here and there, and was turned onto a product that toho makes. it is sort of hard to explain what it is, but here goes - kind of a sheet of metal that is the same size as a 5x7 film holder. there is a 4x5 cut out, and spring back that a 4x5 film holder slides into. it also fits readyload holders too.
( a picture is worth a thousand words )
it looked like exactly what i was looking for, and if need be, i was going to send the whole thing to sk grimes to have them work a little magic on it, so it would accept a polaroid back ( either 545 or a 500, it didn't matter to me since i use both).
i ordered the puppy from badger graphics and it took forever and a day to arrive, but after about 3 months it arrived ( slow boat)
i ended up modifying it myself to use a polaroid back - turns out that i just needed to file about 1/32 inch off of 2 little pieces that are on the spring back because little did i know the polaroid holders are a little bit wider than a standard filmholder or a readyload holder.
if you were thinking about getting a 'proofing' or "reducing" back for your camera, the toho back works great!