You want to be careful with Flexwatt if you're putting it inside the cage. Because the wires can be exposed to moisture, you take the chance of shocking the snakes if they get too much water/urates/etc on the floor. I'd suggest you go with Radiant heat panels instead. You can get two smaller ones if you're going to split the tank and mount them on the ends of the tank on the inside.

Another choice would be something like a small pig blanket (sealed heater that is used with pigs) to put inside the tank. These stay at a low temp and have been used for years for large burms and retics.