Never used tubs before - questions!

[Syfaio]

I've always kept my snakes in glass with screen tops so I'm new to the whole tub deal. It's just two snakes... for now.. (I've got the itch for a few more and designs for a rack!). I've got two sterilite tubs with locking lids, two UTH, and a thermostat. I'm really hesitant about putting the UTH on the bottom of the tubs because I don't want my snakes to get burned. Would putting it on the side of the tub make a good enough hot spot? And what's the best substrate to use in the tubs? I've always used natural looking stuff but that was mostly for appearance.

You can use aspen or newspaper cypress multch really up too you and as long as the uth are hooked up to a thermostat you should have no problems with them over heating