I can't comment on exo-terra mats themselves as I've never used them (I use habi-stat and another brand I can't recall right now) but
You're not really supposed to put heat mats and thermostats inside the viv, but to be honest I set mine up like that when I was a new owner. and had it like that for about 8-9 months. Saying that, the reason you're not supposed to do it is the probe can be moved by a snake, or the mat can get damp and that will make the temperature spike which could harm your snake (I didn't know this when I set my viv up) I did have a problem with my thermostat but that was a different issue as I use that stat now. What size is your wooden viv? It might be a better idea to scrap the idea of using a heat mat and go straight for a ceramic bulb, but if you did that you'd need to get a pulse thermostat as the ones from Swell are on/off ones. Another thing you can do is put a 'fake bottom' in the viv, I have two that have a glass layer so the heat mat is sandwiched between the wood and that.