Question about thermostats!

I have a young corn snake in an exo terra faunarium.

While I was told by the reptile store guy I wouldn't need a thermostat with my heat mat, I've found that the temperature goes up to 37C if unwatched (even with an air gap between mat and faunarium). So I've basically had to act as a thermostat myself, constantly switching it on and off.

Anyway, I've just got a Habistat Mat Stat and I'm wondering where to put it. At first I kinda assumed it would go INSIDE the faunarium with the thermometer sensors (I have one in the cool end, one on the warm end). But there are no holes in the faunarium big enough for the Habistat sensor to fit through. It's really chunky. :s

So basically, should the sensor be inside or outside the faunarium? Where should it be in relation to the heat mat?

Please help out a confused little noob. :|

I am assuming that the Mat Stat is like a thermostat? I would put the probe on the outside and set it to the temp you want it. Check the temp inside the cage and adjust it if you find the temperature inside is too hot/cool, since you can't put the probe into the cage directly.