I've had mine taped with aluminum tape directly to my tub for over a year not without problems. There is some clearance between the tape and what I have the tub sitting on, though (I used a stack of felt furniture "feet" to do it)--because I've read that you don't want to sandwich the UTH between two solid surfaces. I think I have some old pictures kicking around of how I've got things set up. I'm using a larger tub now (Auryn's grown) and replaced the hides with newer, larger things, but the set up is the same.

This first photo is a long-shot of how the tub is set up--notice the little feet to give the tub some clearance.



This is the bottom of the tub--where I have my flexwatt taped with aluminum tape and my thermostat probe taped to the flexwatt with more aluminum tape.



Hope that helps!