I pledged towards the Jollylook camera project on Kickstarter, and they are very close to delivering. They engineered their own Instax Mini developing unit and slapped it inside a cardboard vintage bellows type camera, so it's not an impossible feat to make your own. They somehow had a demo unit before the campaign which I assumed was made from existing parts. I, as well as some others, have been pressing them for an Instax Wide version. This camera is cheap, I got 3 of them for just over $100 with film, so I am going to see if I can fit the mini development unit onto a Mamiya Universal and Bronica SQ-Ai. The wide is no good for that Mamiya as the film will jam upwards right into the eyecup as it ejects. I don't like the Belair back for it since you'd have to eject it downwards which means the thicker part of the border will be at the top of the image once it develops. This is something to strongly consider when making an Instax Wide back as it should eject upwards without obstructions for a photo in the correct orientation. Instax mini can pop out of the side so that's easier to work with.
Alternatively to this, I have already completed an Instax Wide 300 conversion that uses the Mamiya Universal Press mount. What I did was mount the lens onto the smallest macro tube I could find, and jammed that into the lens barrel of the Instax Wide 300. I had to trim a lip, and then sand the inside of the barrel until the macro tube was able to slide in snugly. About to post a separate thread for it so look out for it soon. It is very similar to the custom Wide 300 that ic-racer posted in post #20 above. It's been absolute fun to shoot great looking Instax Wide pictures.