The easiest, cheapest way to do this (narrator: it's not THAT easy, but it's relatively cheap) that I found was to use an Ansco 3A body (3A film, aka 122 roll film, is five 3-1/4" x 5-1/2" exposures on a roll). I put a couple rails in front of the film gate for 120 film width, and bought some 122 to 120 spool adapters from evilBay vendors. I drilled a new hole in the back for exposure numbers, using a red piece from a swatch book for the red window. Using the 6x4.5 numbers, it's a little more than 3 per shot. A pressure plate from a Kodak 1A keeps everything flat-ish. The format is 6cm x 14cm, and you get 5 exposures on 120 film if you're lucky ( there are some winding gremlins that crop up from time to time). The results when it all comes together are spectacular, though.