Quote Originally Posted by Etr420 View Post
More seriously, though, could you do something with a strong, clear ziploc plastic bag somehow glued to the rim of a UV filter? I think it was once possible to buy these.
I think this is the best idea posted so far. My inclination would be to use silicon for the glue, but some experimentation to see what forms a good bond would be in order. You could take a few spares for when the ziploc bag gets damaged.

Even with 645 format, 120 film takes quite a bit of changing, which seems like the weak link in terms of dust getting in. All things considered, I think I'd be looking more at 35mm in your position, maybe with an EOS body that can function as a p&s---it'll take your Canon lenses, you don't need access to anything but the shutter button, and you can change film *fast* with the ones that auto-load. They're also cheap enough that you could treat the body as a disposable---I once got a Rebel X for six dollars, and I currently see some older EOS bodies as low as eight bucks at KEH.