I'm in the market for the same thing. I knocked my cap off of my seat with the fuel hose last weekend. Of course like a jelly sandwich, the chrome top hit the pavement, dented and scratched....darn! I had to look at all the way home.
One of my students told me about the medallions. I'll buy one tomorrow at the dealer. We have a big ride (Back Roads Classic) and lunch planned for tomorrow leaving from and returning to the dealer.
I'll probably use clear epoxy but I'll make a shield out of masking tape to adhere to the cap with a hole just slightly larger than the medallion prior to using the epoxy to keep the cap clean and free of any excess epoxy. If you do it right, the epoxy will last 4ever. Also, I like to use acetone as a cleaner prior to gluing. That removes any oils, waxes.