I think it depends on how much "from scratch" you've agreed to. I've never done it, but I would imagine building your own instant packfilm holder would involve a tremendous amount of trial and error - specifically because the rollers need to be precisely aligned for them to spread the "goo" from the pods correctly and evenly when you pull the exposed shot from the camera.

If you're allowed to buy a shutter (as opposed to building your own guillotine-style drop shutter), I suggest buying a cheap Polaroid Automatic Land Camera from eBay, etc and removing the film holder from the back. A lot of the Automatic Land Cameras go cheaply because they need batteries that are no longer manufactured and have no manual exposure settings. You don't need the more expensive models if you're just after the back (some of the lenses vary in quality - plastic vs. glass, etc, but irrelevant for your project).

Also, if you're looking for a cheap shutter/lens combo (and don't need flash sync), I suggest looking into the Kodak 1/1a/2/2a/3/3a folding cameras from the early part of the 20th century. Most (all?) of them used a simple ball-bearing self-cocking shutter mechanism (as a result of their simplicity a lot of them still work), and most of the the formats they were designed to cover were of a similar size or larger than the current packfilm standard size. I own a couple of these folders, and it looks like the shutter/lens assembly can be removed without a huge hassle. The simple "Made in Rochester" ones are the cheapest and should suffice for your needs.