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

Update: after only a week, Plasti-Dip has proven to be even more bullshit than Sugru.
@jwz Quick, non-destructive thing to try: Scotch self-fusing silicone rubber tape.