Belly heat is the most commonly used method. If you'd rather do that you can't go wrong. Back heat works, it can just be a little trickier to get right for some people. For tubs most people use Flexwatt. It's so thin it doesn't really effect the height. A UTH might cause problems. I would leave a 1/8" gap above the tubs without the UTH, or more if you use spacers. Just be careful it's not a big enough gap that a snake could escape.