i would actually go with heat tape for a rubbermaid. (less chance of meltage) Its all about friction. The electricity somehow causes friction which lets off heat. (i found out a while ago but i cant remember) If you buy it in the pre pack, (all of the chords, clip on covers, etc etc etc...) all you will have to do is sauter on the wires from this chord, tape up the end, put on the plastic covers, tape on, plug in, wala, WARMTH!!
Oh and either way you will need a thermostat and a thermometer to monitor temps. With the thermostat, you plug your heat tape into that, then you somehow put your probe into the cage, plug that in, and your done. (don't ask me how to get it into the cage, im not sure with a rubber maid) on second thought, you could put it directly on the heat tape, but that would not factor in how much heat it loses going through the plastic. Now with the Thermometer, most of them are batterie operated, so all you have to do is just put the probe inside the cage, or on the heat tape, and your done!