Towels absorb moisture, you want to put something that repels it. Like aluminium foil, plastic wrap, plexiglass. Even though your temps. are good, I would get some belly heat with a thermostat. It's better for the snake and it helps with the humidity. You can get an under the tank heater or get some Flexwatt heat tape, whatever, just make sure you get a thermostat and plug your heating device into that. I use Aspen bedding, but all my snakes are in tubs, except the cornsnake.
The preferred method is tubs with belly heat. But I know you want to display your snake and thats why you use a tank. You can use a tank, but you should try and set it up like most of are saying. Trust us, we have been there.