Put your thermometer probe inside on the Floor of the enclosure above the heat mat.. Thats what you want to know, how hot the plastic floor is where the snake lays above the UTH.
Your going to have a hard time using that enclosure.. You'll have to try and drill a hole in the side to run the probe under the UTH above the substrate.
PS- if you touch the UTH and you say it feels warm them its not 70F. Your body is 98 so if the UTH feels warm to your 98 degree body then its sure over 100.