Well I do have a cooler that I put a 15 watt plant heating pad in, controlled by a thermometer and I can maintain up to 30 degrees above ambient inside it. It works better if there's high mass objects in it to serve as a flywheel for thermal energy.
But I'm not sure if it's safe to put flammable liquids in it.
The thermostat is electronic and it's outside the cooler with just a probe inside. So no worry about sparks from that. But I don't know if the Chinese heating pad would make a spark when the tiny heating element wires inside it eventually short out.