Clear Filler for inside of bottle caps?

Along with a recent purchase from Etsy the seller included a bottle cap with a magnet on the back. The bottle cap has a design on the inside and she filled it with a laquer or some sort of clear filler. Anyone have any idea what that filler would most likely be?

Most likely either slow-cure epoxy or clear casting resin. Casting resin is available at most craft stores, and epoxy is available practically everywhere. Slower-setting epoxy tends to be clearer and less yellow than the 5-minute variety.