Instax Wide backs for the Mamiya Universal Press and Polaroid 600SE cameras were readily available, until they weren’t.
If you’re browsing eBay, the equivalent of an Instax Unicorn has surfaced on the web: an Instax Wide back for the Mamiya Universal camera. This adapter lets you load up Fujifilm Instax Wide film and shoot it with the medium format lenses used by the system. In addition to that, the system gives you full manual control over the image. With that said, you have full manual control over a modern Instant film format–currently the largest available on the market to consumers. Serious portrait photographers will adore the ability to do this with a camera that can fire off flashes and do so much more. The only other cameras that is nearly this versatile is the Mamiya RB67 and RZ67 series of medium format cameras.