Not a rubber mat, those things smell..

There are alternatives. Do you heat from above or below? Makes a difference.

Heat from above:
Reptile carpet (or outdoor carpet make from recycled polyethylene bottles)
Polypropylene sign material (corrugated plastic "cardboard" material)
Sheet vinyl
Sintra (closed cell PVC, can find online. The 3mm thickness can be cut to size with a straight-edge and a ultiliy knife)
FRP
I forgot, but it comes in a 4ftx8ft, white flexible plastic near the FRP stuff

Heat from below:
Reptile carpet
Sintra