I agree flexwatt is the way to go. I notice that most pads being same make and size will vary in temps somewhat and can cause a problem contolling equal temps to each level. you might be asking for a big headache. one problem with the flexwatt is it's so thin and the coating is plastic, sliding the tub over it again and again will make it wear and also cause problem. so I would use very thin metal like roof flashing and cover the flexwatt with to protect it from the sliding tub. just make sure the ends where you cut the flexwatt, the metal inside the flexwatt doesn't come into direct contact with the flashing.