Use epoxy to glue the inside center of the bottle cap to a tall rivet on the belt. You may have to remove a bit of plastic/rubber liner inside the bottle cap to get a good glue joint. Big pop rivets might be tall enough and smooth enough on the back. Punch holes in the belt and use massive amounts of hot glue pumped threw the hole and into the bottle cap. You might be able to avoid the holes in the belt, but I'm not sure how well the glue would stick-depends on what the belt was made of. If you don't mind damaging the front of the bottle cap, you could punch or drill a hole in it (or more than one) and sew or rivet it to the belt. If you punch holes in the edge, you could use them in a chain-maile style belt. Metal or plastic bottle caps?