With 4x5 inch size, it becomes possible to consider roll-film - you can still get rolls of 4" wide b&w film and perhaps color emulsions as well. You'd have to build in some kind of transport mechanism for the film and a way to make it light-tight along with being water-tight. If 8" or 10" (9.5" or whatever it is) roll film is still available, I don't think I'd want to know what the weight of something like a roller transport mechanism for that would be. The motorized back for my RB67 is heavy enough, and it only needs 4 AA batteries!