Let me be the first of many to reply and try to help you. Welcome to the site, this is a great place to get some guidance.

Fist of all, Pictures are great to help see what your set up looks like. Your humidity problems are probably from the overhead light. Is the light your only heat source? I would strongly suggest getting an Under Tank Heater (heating pad made for reptiles), but make sure you use some sort of thermostat or light dimmer to control the temperature, as they can get very hot and burn your snake.

Second, you will need to throw away the analog thermometers, as they are garbage. Go to Walmart or Lowes and get a Acu-rite, indoor/outdoor thermometer. I just got another one from Walmart for $12. It has a probe on it so one end can be on the cool side and the probe under the warm side hide to better measure temperatures. The Acu-rite model is grey in color and has an outdoor indoor reading on it.

Humidity will suffer due to the screen top as well. Cover 3/4 of the screen with tinfoil, or plexiglass or something. Leave a whole (on the foil or plexiglass) for the light.

Other tips for keeping temperatures up is insulating the sides with Reflextix insulation (found at home depot or lowes) and or cork board or foam board from a craft store.

You want to aim for temperatures of 90 to 93 on the warm side on the substrate and 81-83 on the cool side. Ambient air (air in the cage) should not get much lower then 80 with optimal temp being around 82.

Don't worry about the cage being cramped. If your ball is a baby or fairly small, they love cramped area's. I actually put newspaper balled up with several hides and plants to clutter up my tank.

I've probably left some things out.... but I'm sure someone will fill in for me. Good luck.