Make sure to measure the temps ON the floor, since that is where your snake will be 99% of the time. Be careful about putting anything inside the snake enclosure that is sticky. Any kind of adhesive WILL eventually end up on your snake, and then you will have to wait for the next shed to get the thermometer off him!
A human heat pad on low will be fine. If your snake needs the heat, he will perch on top of it. If he needs less, he will move off it. Air temp is not as vital as floor temps (although it is certainly NOT something you should ignore!)