I've had some issues with heat mat's. The temp controller is definitely a good idea but I've found them to be uneven with hot and cool spots. Even with the controller, it needs to be in the right place.
I once used rigid insulation as a buffer from a cold concrete floor and this happened. They shouldn't even get this hot.
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I still use them for stuff but now I use several layers of corrugated cardboard with no plastic on both sides.