Here's a little fun-cam:
I ordered a Lomography Diana F+ instant back, to mount onto one of my (many) old 620 film TLRs
first I removed the Diana back,
then removed the back door on the Argus ( I chose this one because they looked like they'd match up perfectly)
Then, some high tech black duct tape..... a few light leaks at first, but then I removed it and re seated it - F8 indoor is not the best place to test, but the results look promising!