Yeah those pads work pretty well. Propagation pad? Heat mat? They don't get very warm, I think they are mostly designed to sit under some propagation trays, or grow cups.

Insulation IMO is probably the best bet. The more the better, but that creates a higher need for ventilation. For example, I have a nice triple wide Super Good Cents home, and we would suffocate if we did not run a whole-house fan.