A better method then using tape would probably be to adhere the thermostat probe to the bottom of the tub, and then hot glue the thermometer probe inside the enclosure. By doing this you can monitor the actual temp they are sitting on way more accurately. You never want the thermostat probe not adhered to the hot-spot because this will cause it to turn on when it shouldn't. If she moves it to the other cool side of the cage the thermostat will turn it on and who knows how hot it will get. I used monitor it on the outside with a thermometer probe right next to the thermostat probe, but realized (after dr.del pointed out) if you hot glue it to the inside its so much easier and accurate.
Also, when your adjusting your tstat, use your thermometer probe as your reading. Once its turning off at 90-92 going by the thermometer your golden. This might mean the tstat is set to 95 to make this happen. Being that the tstat probe is on the outside it will be getting a cooler reading, so if you turn it up your compensating for the cooler reading. But by having the probe on the inside you will know exactly where to set it. Just takes some fiddling and patience, good luck.