Let Friction Ring.

Dear Lazyweb, I have this pulley wheel, 50mm inside diameter, 4mm groove. I need a rubber traction ring to go inside it. I cannot find anyone who will sell this to me. The ring must be flat or concave, not round like a typical...
https://jwz.org/b/yk33

@jwz Misumi sells configurable pulleys intended for round belts, with vee grooves for non-slip, e.g. P/N SL-MBRDA45-2-P8. It’s pricey (US$22). Search on Misumi Components (they’re worldwide).