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The UTH you have should be running 24/7 and be controlled by a reptile thermostat.
How are you measuring temps? If you aren't using a digital thermometer, chances are the temps and humidity are off. Home depot sells a cheap indoor outdoor thermometer and hygrometer combo that works great for single setups like this. Its called an Accurite and is only about $12. If you are not controlling your UTH with anything, pick up a lutron lamp dimmer at the home depot as well ($11).
If after you control the UTH and have it running constantly and are getting correct temperature readings and the temp is still too low, then I would think about getting a second UTH for the cool side or a lamp (these do lower humidity though).
Also, you could insulate the back and sides of the tank with black foam board. This will help keep in the heat a bit better. Ditch the towel on top of the tank and replace it with some aluminum foil tape on the screen. Cover about 3/4ths of the screen.
I would also think about getting the snake a second identical hide for the cool side.
Good luck!