The zoo-med under tank heaters say that they are to be used only with glass. I consider that to be to some extent their way to cover themselves in case there are problems. A heat mat used without a thermostat or rheostat could melt the plastic. I've had no problems using heat mats with thermostats under plastic. If you think about it, most racks are plastic tubs that are heated with some kind of heat mat. It is very important, though, to use a thermostat and make sure it's working correctly.
Aliza