What you can do is:
-Buy a large plastic "tupperware" bowl, or something similar. I got mine at the dollar store.
-Using an electric heat knife (or something else), cut a whole in the lid a bit larger than the width of your snake. If you're using just a regular knife or something sharp, be sure to file down the edges, so they don't cut the snake.
-Buy terrarium moss from your local pet store. Stick some in the bowl, dampen it up with water, and leave it in the terrarium.

This is a nice way to make a cheap "humidity chamber" for your snake. If it's feeling too dry, it'll make its way in there.

But agreeing with the other guy, for ball pythons:
-Plastic is better than glass.
-Heat pads, or even better, Radient Heat Panels are better than bulbs
- Plastic tops are better than screens.

I'd check out Animal Plastics. The biggest downside for their terrariums is the EXTREMELY long order processing time. They make a good product, and are always overbooked. It might be worth putting in an order ~6 months before you think you might upgrade. 20 gallons is plenty for a new snake, but you'll want bigger once it starts reaching adulthood. They'll also install a ProMist RHP for you, which I'd recommend. They're great at keeping temps right and humidity in.