+1000 for the Gorilla Glue Epoxy. I use that stuff for all types of home repairs, especially anything to do with wood or plastics!
When I got the wheels I have now, the Mustang center caps sat all the way into the center hole. However, the outer lip of the Mustang caps stopped the cap from looking counter sunk by a half inch into the wheel. That lip made the center cap flush with the hole in the wheels. So I just took off the tires, sat the Mustang center cap into the hole, taped it to stay there and used a high strength glue stick and glue gun I bought at Lowe's to glue the caps into the new wheels.
I've driven on them that way for about three months now and haven't lost one. They aren't loose at all either. This has been through rain storms, 140mph blasts and the cold of winter. The only thing that sucks is that any time you have to get the tires balanced, the caps have to come out. But that glue comes off the caps pretty easily.
Soon, I won't have to worry about it though. I'm going to buy a set of RTR's as soon as they're back in stock.