If you have had rubbermaid/iris/sterilite containers melt while using undertank heating, then there must have not been any regulation on the heating device. That is extremely unsafe for both the animal and your house. Plastic has quite a high melting point, so the heating device most likely shorted out and melted everything.

I use two different sheets of flexwatt controlled by a Herpstat 2 and the bottom of my Ball's tub is warm, which obviously isn't even warm enough to make the tub warp.

Every single, reputable Ball Python breeder I have seen uses plastic housing in racks and undertank heating such as flexwatt to heat their snakes. There is not much of a risk if you have a good thermostat controlling it.