I place the Stat probe between the heatmat and the bottom of the enclosure (outside).
That way the snake can't mess with the thermostat readings by moving it, peeing on it, pooping, etc.

Because the heat has to travel through the bottom of the enclosure you might need to set the temp a bit higher on the thermostat to reach the proper temperature inside.

Btw, check the temperature on the surface of the enclosure and not on top of the substrate since your snake can (and probably will) move it aside and be able to touch the bottom directly.