Good looking snake.

How are you heating the tub? What is the black wire for?

Having wires going in from the top is a sure fire escape area for these masters of escape. You could drill a hole in the side just big enough for the sensor. Don't apply too much pressure as you drill or the plastic will crack. Holding a piece of wood on the opposite side of the plastic will help it not to crack as you drill. Some people also burn the whole in the plastic with a soldering gun.
Use a hot glue gun to seal the hole once you have the sensor placed.
Just an idea.