It's not hard to put a 4x5 back on these, (plenty of threads here about it actually), and then you can use any graflok back, including the ones for Fuji FP100c. I am doing this exact project, and it's mostly done (4x5 Graflok back on 3x4 RB Super D). Image size will be about 4 x 4.25 or 4.5