Get someone to help you, take out the snake, clean out the "furniture", flip the tank over, peel the UnderTankHeater back, and tape the probe to the center area of where the heat mat will be. put the heat mat back, use tape or not as needed.
If you are worried about the probe getting "crushed", you can put the tank back on top of a thin, soft layer (foam, metallic bubblewrap insulation, cardboard, you get the idea.)