They'll be perfectly fine as long as you control it with a thermostat, and as was said above you're better off keeping it to one half (ish) of the tub so the snake can choose whether it wants to be warm or not.

If you use tape to stick the UTH on (which I recommend because then you can un-tape it and use it on another enclosure for whatever reason) make sure you use aluminum tape because it's a lot more heat resistant than the plastics which could soften up over time with the heat.