If the mat is in the viv, what is between the mat and the snake? Those mats get very hot and could burn or kill your snake. What are the temperature readings you are getting now with the thermometer you have now?
You may want to get a tile that is as big as the UTH and put it on the UTH, but I would also get at least a plug in lamp dimmer at Home Depot ($10) if you do not have the money for a thermastat. This will regulate the temperature. This is the set up I use.