Generally I don't like to use a heat pad with a tort. Heat pads are for snakes. Torts have claws and like to dig down; if they do that into a mat, it is a big electrical safety risk. Torts need their heat from the top, not from underneath.
Is the heat pad sold as safe for outdoor use even?
"This heater must be used in a covered cage/enclosure
DO NOT expose this heater to direct sunlight, rain, or high humidity levels (over 90% Relative Humidity)"