I jury-rigged mine, making a simple mask out of aluminum flashing that had been cut from a roll (the "memory" curve acts like an additional flattener, keeping the film flat against the pressure plate.) For the rollers, I hot glued some fiber washers onto the one 116 reel I did have, centering the film take-up. At the other end, I cut a plastic 120 reel into two parts and placed them, cut side out, into the film recess. They press fit pretty easily. the space between them is just sized right for 120 film.
Advancing the film is a bit of a guessing game. I went through a sacrificial roll with the back off the camera and found that, if I center the start arrow of each new roll before I close the camera, I can advance 6 half-turns to the first exposure, 6 again to the next, 5 and 5 for the next two and 4 to the last. It's rough, but it works.
Here's a (thoroughly uninteresting) example taken the day after I rebuilt the thing.