Best way to keep a belt drive from slipping is to use a pulley with teeth. But if you need a custom belt, mold the teeth out of epoxy. And be sure to use mold release on the belt.
The most critical part of doing this is making the pulley the right size to make the belt ends come together without extra space. So I do not know if it can be done on a rim pulley.
Spoked V-belt pulleys can be found very easy in many diameters, and its a cheap experiment. Epoxy doesn't like metal much, but If the metal under the teeth was scored with grooves by some kind of engraving tool, like a Dremel to give the epoxy more "bite", or make sure the epoxy goes into holes, it will last longer. Maybe even fiberglass cloth may help.
If you're going to bolt tabs on the rim to hold it in place, do that first.
|Idlers are essential|
|very cool bike to motorize|