I'm doing an Instax SQ back for my Hasselblad 500CM, using the processing unit from an Instant Magny 35 (Kickstarter that came up with a very nice Instax SQ attachment for Leica M, Nikon FM, and other cameras). I got a spare processing unit from those folks, and will glom the back attachment mechanism off my old Polaroid-Hasselblad back (useless now that there's no film for it anymore).
Milling down the processing unit to get the film plane as close to the original Hassy spec as possible is the tricky part. I'll likely have to be content with a back that doesn't focus all the way to infinity unless I come up with a creative way to cut another 1-2 mm off the depth of the back. The Hassy body will NOT be altered at all. I should at least be able to make a nice little distance strip for accurate focusing and affix it to my Hasselblad lens focusing rings.