I have an abomination that I printed years ago that adapts an Instax SQ1 to Hasselblad lenses (and has an RB67 focusing screen on a viewfinder next to the Instax for composing and focusing).

It's sooooo impractical but the results are so fun.

The process is to compose, focus, put ND filter on, swap from finder to film body, half-fire the shutter, pull out the dark slide, fully fire the shutter, push in the dark slide, eject the film, rewind the shutter, and go back to the finder.

Watching Kyle McDougall[0] take his medium format photos is a soothing experience (but I know full well I would not have the patience for medium format.)

[0] https://www.youtube.com/@KyleMcDougall/videos

I’ve been wanting to build one of these for awhile. Did you follow a plan/blog or design your own? I already have donor instax and Polaroid bodies not sure what to put it as a back on