This is a fire waiting to happen!

My suggestion, if you are going to keep the bins on the floor as it looks in the picture.

1) Get heat tape
2) Pick up some cheap ceramic tiles to put beneath the bins (between carpet and bin)
3) Get those tiny rubber/silicone bumpers you might use on a cabinet/drawer that slams shut
4) THERMOSTAT!

Place the heat type on the ceramic tile, place the bumper on the tile so the bin is just above the heat tape and not right on it. Put thermostat probe in that space. Should be no more than 3/8-3/4 inch depending on size of the bmpers

DO NOT FEED THE ANIMALS until you have the right temps.