Hello to the forum!

I was wondering if someone here could help me out with a little DIY project I am doing. I took up a bet with a friend of mine, where in 30-days time he assembles out of a pre-existing camera, and I build from scratch, a medium format camera that takes instant film (we settled on FP-3000B), and we see who can come to it the cheapest. Now, I already have my eyes set on a design, and I already have a modified (old, broken) Sigma 135-format lens that covers my desired format of 6x7, BUT, I have no access to, or possession of, an instant film holder for the film. As they cost enough to break my bank (as I still have a shutter to buy), I was thinking about constructing one from scratch, or rather, having the holder be an integral part of the body of the camera.

Has anyone attempted anything similar and/or does anyone perhaps have any drawings of the workings of a instant film back? I'm trying to figure out the function of the rollers (or absence thereof) in certain backs from images and online tutorials, but I'm not getting anywhere, having never used instant film before.

Thanks in advance!