DYI replacement for wiper blade cap?

My '94 Toyota Previa has aftermarket Anco brand wipers. Each blade has a cap at each end to hold in the insert. One of those caps has broken off, so the insert works itself loose, starts flapping around, and would eventually fly out. I have to pull it back into place after every short trip, and a long trip would mean frequent stops just to pull back that insert.

Anco says they can't sell me a replacement cap; I'd have to buy a whole new blade. But the Previa was designed with "wet" wiper blades, meaning that the windshield washer fluid is routed through them. That makes for expensive wiper blades.

Any ideas on how I can fashion a replacement cap myself? It doesn't have to be pretty; it just has to hold in the insert and not scratch the windshield.