Isaac Blankensmith writing in PetaPixel about building an Instax instant film back for a Hasselblad 500:
Instant photos are magical. They develop before your eyes. You can share them, gift them, spill water on them, draw on them. The only problem is that most instant cameras are pretty cheap — that’s why I’ve always wanted to hack my medium format camera to take instant photos with shallow depth of field and sharpness.
Since Fuji ceased production of their Polaroid-compatible peel-apart films a few years ago, there has been significant interest in a Fuji Instax back. At least two Kickstarters have been announced, Hasselblad Square Instant Film Back and Rezivot Instant Film Processor, but neither of those was successful.
@instantmediumformat on Instagram converted an old Mamiya Press Camera, but has offered few details for those who wish to follow. Blankensmith’s post is a pretty good starting point for people who’ve been considering building one for themselves–possibly people like me.