THIS.
Also... Do not EVER use ANY kind of "heat rock" for snakes...
Your tank needs to be insulated, heavily, from the outside to avoid the snake coming in contact with adversarial material, and to keep the heat and humidity in. You also need to get a thermostat to regulate your heat pad. OR... you can get an enclosure specifically built for snakes and NOT for fish. There is a reason why they are called "fish tanks" and not snake tanks.