Thanks for the tip! I'll check them out.
I'm working on a cardboard cartridge that allows me to use a normal 35mm cassette by unrolling all of the film into the supply side and then winding back into the cassette. In the X-15F making two notches 180 deg apart in the cassette spool will allow the film to engage the winder. IDK if that'll work for any Instamatic. The Legacy Pro worked out OK today but I still need to get the winding distance correct. Have some Superia 200 in it now. There's a light leak around the top but I believe it can be dealt with using some felt.
Also looking at a few examples on *Bay that come with film included for reusable cartridges.
Tell you what, though, how nice to just wind and snap like the real old days! No batteries, no menus, no focus - just wind, compose and click. Yeah, some of my pictures didn't "get" but so what? They didn't back then either.
Didn't Einstein say something like "any fool can make something more complex, but to make it simpler is the real genius?" Sometimes perfection ain't what it's cracked up to be.