I just went through this. Changed the environment and husbandry for my snake and she is much happier.

What I learned:

1- 40 gallons might be too big, these snakes like smaller spaces, 20 gallons is about as big as you need to go and is big enough for an adult.

2- A towel on top of the tank will not cut it. I tried this for awhile and the humidity would plummet to like 38% in a matter of an hour. Neatly tape and tinfoil the top of the tank leaving a cutout for your lamp if you need it. I also learned that CHEs suck and I get a LOT better results from an infared heat lamp on a dimmer to control it.

3- Clutter is good, put a lot of fake fauna in the tank, it'll make the snake feel more secure.

4- Hides are REALLY important. Closed, smallish and dark. You want the snake to take up almost ALL of the space in its hide and feel safe in it. I was using half logs but they do not fit that bill at all. I ordered two medium sized hides from reptile basics and they are perfect.

5- Moss helps humidity a ton as well as a big water dish. My dish might be TOO big but I'll address that later, for now it isn't hurting anything. With a big dish, the top screen covered with foil and a few patches of terrarium moss you should be able to keep your humidity level holding steady where you need it to be.

Here is a pic of how I just got my setup and everyone on here who saw it said it was a good setup for my ball. This is a 20L tank.