I just bought this Tektronix C-53 osciliscope camera so I could take the polaroid back off the thing and use it for a similar purpose.
While this seems to be working out well with no hacksaw required (you just uncilp the back and even if you just put a pinhole on a flat card on the front of the back then you get a focal length of about 45mm and it works), it sounds like it's not the absolute cheapest option.
Looks like the colorpack is going for at least $10 + $10 shipping on the US eBay if you don't want to spend time searching for one.
Here's some documentation that might interest you:
See the instant pack film guide as it explains a lot of things.
After taking apart a roller assembly for lubrication and putting it back together, I'm quite sure that you don't want to try to make this part yourself. The rollers need to be perfectly smooth to spread the "goop" evenly, they need to be connected with heavy springs that force them together with enough force to spread the goop while allowing them to move apart just enough to allow the plastic shims at the end of the film (where the excess goop reservoir is) to push the rollers apart to avoid squeezing all the excess out inside the film pack, and there seems to be a guide on springs that I don't totally understand the workings of.
Being the least bit off on any of that will likely cause you to waste whole film packs at a time (as I have with poorly lubricated and dirty rollers in the past).
If you want to make as much of the camera as possible I'd at least buy a roller assembly, but trying to buy just a roller assembly and nothing else will probably be more difficult and expensive than just buying the cheapest pack film camera. (Only one I see on the net was bring sold for 35 euros and I'm not sure it was even the right type.) Trying to make your own holder for the film cartridge that's precise enough to align the film with the rollers might be pretty difficult as well.
Essentially what happens when you pull a film sheet is that the little tab pulls the negative from the front of the pack and around the opposite side, flipping it over and pulling it under a sheet of paper. At the same time it forces the larger tab between the rollers so it sticks out as soon as it's lined up properly. My guess is that even if the rollers are perfect, the springy guides or something align the negative and the paper and provide just enough resistance to allow the glue on the little tab to come loose once the sheets are exactly aligned. Then you pull the big tab which squeezes the "pod" of developer reagent goop so it breaks and spreads it evenly between the sheets. If not spread evenly then you get bands or missing parts in the image. At the end of the sheet, attached to the negative side I think, are some little plastic things that are thicker than the sheet. These push the rollers apart so that they stop squeezing the goop and allow the extra to just accumulate between that little strip that's last to come out.
Even if you have ridiculously expensive precision machine tools you probably wouldn't get it right the first time. The Hasselblad, Mamiya, and Tektronix backs that I've seen all look like they were probably made by Polaroid and perhaps, at most, glued to some bits by the camera makers to attach to their cameras.
Oh yea, NOTE: The older film cartridges were made of metal, and the modern Fujifilm ones are made of plastic. The back door of these old film holder backs usually have this metal strip spring that pushes down on the film pack, but it may put too much force on the plastic pack, bending it inward and causing jams. I was having this problem with my Polaroid 250 until I read about this somewhere and ripped the spring out. (You can just bend it until it breaks.) After that it was much easier to pull the film out and I wasn't having jamming problems. I also wasted a whole pack of film in this tektronix back, probably because I didn't pull those springs out first. I'm not sure if the back springs are supposed to keep the pack from rattling around or what, but I seem to get good results without them using FP-3000B and FP-100C. I haven't noticed any problems after removing the springs so I'm not really sure what good they were to begin with.