The rubber came off my O3DS circle pad, and I bought a pack of plastic N3DS circle pad covers (Cyber Gadget?). It seems the original circle pad is very slightly larger, so I *replaced* the rubber with it. To avoid glue dripping inside, I attached it with permanent gel glue dots rather than liquid glue. I have another cover from the same pack over my N3DS circle pad for better grip.
My O3DS might be harder to sell now, as the glue dot isn’t pretty, but the circle pad works fine.