Hey Tracy,
When I used my thermostat with the mat in Ernie's zoozone, I had the mat on the outside and then just put the probe through the bars at the top. To be honest, I find it better though without the thermostat. I hav a thermometer over the end with the heatmat anyway.
With your viv, you might have to put the heatmat underneath or if possible drill 2small holes for the wires.....
Let me know how you get on Hun xx
Xoxox