[Finster]

Neat project looks great so far. I had an idea about the internal gear ring. What if you cut a circle the outside diameter you want and put some sheet metal or plastic on the edge so it sticks up the thickness of the gear you want. In the middle of this circle fix a large regular gear that gives the ring gear the right spacing and teeth. So basically make a temporary mold and then pour some strong epoxy type stuff it the space.

Hopefully my description makes some sense, if not I could do a quick drawing for ya. Not sure if this is a better idea then your chain idea, but just throwing it out there. I look forward to your updates.