Well....You know.....I am thinking of buying a broken 3 1/4" X 4 1/4" double dark film holder that is broken on ebay for 8 bucks for research.
Somehow at this time I don't see much point in investing 8 bucks for this broken film holder to use for research to produce the "304" film back.
Perhaps you folks on this forum could show a bit more enthusiasm toward a project like this???
I have just finished one. I used the base plate from a film pack back, drilling out the pins that held the plate to the box that was meant to house the film pack. Adding a 3/16 thick frame of wood to the base, I could then glue on the Polaroid back from some version of the M4 or M3 copy cameras that were being thrown away all over the place a few years ago. Pretty good fit but I had some problems with light leaks. All solved now, I think. I went through a full pack of film testing it, though. So much for wild-ass guessing instead of careful craftsmanship.... Ah well, if you're going to be dumb you gotta be tough. I took the loss and moved on.
Focusing on a ground glass placed in the film holder allowed me to shim up the GG until that was also in focus. Not surprisingly, it needed just over 3/16th of additional distance from the mirror.
Now to go out and actually use the thing.