Not sure where you're at, but try places other than pet stores for a thermometer. Hardware stores (Home Depot, Lowes, etc) have infrared temp guns in the $20 range that work perfectly for our application.
Also, just so you're aware, BPs are notorious for being able to find their way under their substrate (either burying down in bedding, or "tucking them self in" under carpet or paper towels). I wouldn't be comfortable with just a layer of cage carpet between your snake and a heat pad that's not being controlled by a thermostat. If you can move the carpet, chances are your BP will too.