I have Flexwatt taped to the undersides of the tubs. I also have a human heat pad just laying on the top of the upper tub since I wanted to make sure the possibly preggo female is warm enough.
You could probably tape human heat pads to the bottom of a tub. Just make sure to use a thermostat in case the heat pad burns out or starts getting a hot spot (two of my old human heat pads burnt out in the past year, fortunately they both just quit though). It's kind of frustrating - I'm using Flexwatt more now because human heat pads now come with so many safety features (like auto shut off etc), that they are useless for reptile heating.
I have flexwatt taped to the bottom of the tub - but have a thermostat probe taped right againt the bottom of the tub first. So the thermostat is sandwiched between the flexwatt and the tub. Once I got a good temperature inside the tub, I noted where the temperature setting on the thermostat was (it reads waaaay warmer than the inside of the tub for obvious reasons, since it's right against the flexwatt). It's been pretty good so far.
I have reflectix taped under the flexwatt too, so heat doesn't leak out of the bottom.
