Go to walmart and get the Acu-Rite indoor/outdoor thermometer with humidity reading. (http://www.walmart.com/ip/Acu-Rite-I...mometer/896347)
Use hot glue to glue the outdoor temperature probe to the hot side of your tank. Then, hot glue the wire along the bottom edge of the glass leading to the cold side of the tank. That is where you will place the actual unit. The probe that's glued to the side of the tank will read the hot side, the actual unit will provide the cold side readings, and will also provide humidity. Here's a picture of how I have mine set up
It's hard to see, but the temperature probe is hot glued to the inside wall of the tank on the hot side.
Here it is all set up
I stress using Hot glue because using anything with adhesive on it will stick to your snakes causing a bad situation.