quick tub thermostat question

I'm setting up a tub for my snake and was just wondering exactly where the probe on the thermostat needed to be? does it go under the uth (between the plastic and uth) or on the outside of the uth and just tape it on there?

Not sure if it is the same as heat tape but I was given instructions to put the thermostat probe directly on the heat tape by rich from Reptile Basics. But I wouldn't see how there would be much of a difference if you did the same. Just might have to set the thermostat a bit higher to get the desired hotspot inside the tub.

I just taped mine on the the heat tape REAL good. You definitely don't want it to move.