Water Heater Protective Tub (For Aquaponics Sump)

I have an aquaponics system in a greenhouse. Occasionally, the sump tanks will freeze or get excessively cold for my tilapia and freshwater shrimp. If I put a common aquarium heater in the sumps to keep them above freezing, and then the water recedes to leave them uncovered, they will either shatter (glass type) or melt things (metal type).

This protective heater tub keeps the heater submerged when the sump temporarily runs dry. The retained water keeps the heater from overheating as long as