This was for a forum swap - I made it with 2 colours of Friendly Plastic melted into an Artmould. When it was cool I flipped it over and reheated it, and then stamped into it. As long as you wait for the plastic to cool before you remove the stamp, it won't affect the rubber.
Then I pierced it with a needle tool, and threaded it with wire and beads, finally adding a shrink plastic heart, and a few other embellishments. The key was also made with an Artmould.