I'm planning on releasing the STL and instructions when I finish it up, but I've been lazy about it.
* The mini 9 body needs to be cut down a lot to fit. Everything needs to be cut to be flush, or the film will be too far from the focal plane. The holder for the spring for the eject mechanism needs to be snipped and attached closer to the body, or it won't fit. Part of the motor mechanism will stick into the RB67 body.
* I'm driving the motor directly with a LiPo battery. Instax PCB needs to be ditched as it won't fit. If you want to use the Instax PCB to drive, you can bypass the logic by desoldering flash cap and bridging the resistor divider that senses flash cap voltage (the trimpot by the flash cap) to V+, and then bridging the bottom two wires on the right side of the PCB (detects whether lens is extended).
* I used these screws for the RB67 mounting posts: McMaster-Carr part #91771A849
* I reused the springs from the Instax for putting pressure on the back of the film pack. If there isn't enough pressure, the last couple films won't eject.
* Make sure you leave clearance for the RB67 pins to poke out as well as clearance for the finder to flip open
* Use a caliper to measure distance from back of the RB67 body to the film plane. IIRC it was around 7.5mm.
* I didn't bother making a mating light trap and just made it sit flat against the back of the body. Haven't had too many issues with light leaks so far.