I needed to get the exact geometry off of a rubber part and used polymer clay to do it.

The Sculpey Premo clay I used has the consistency of warm beeswax and is pretty easy to work. Once fitted to the rubber part, I put a backer piece to keep it from torquing and fired it at 275F for 30 minutes.

The result is a very strong piece that can be sanded, polished and painted.

Here's the rubber piece for the impression. The powder on the surface is sodium bicarbonate which is used as a release agent. The powder is quite fine but the camera makes it look a lot more granular.

Here's the clay squished into the rubber. The clay is soft enough to conform without distorting the rubber. If it does distort a bit, just let it sit and the tension in the rubber will move the clay into the proper orientation.

The backer plate stabilizes the geometry and also becomes a base for when it's fired. I used PVA as a release agent on the metal.

This is right after pulling it from the rubber. Note the detail. The black spots are some grease on the rubber. It won't affect the finished piece.

Once fired, the material is strong, tough and pretty hard.