If you're having trouble with humidity, put the water dish directly below the heat lamp. You may need a bigger dish or need to fill it multiple times a day. I would also recommend putting aluminum foil over the screen instead of the towel. You can fold the foil over the edges of the screen so it stays on the screen when you remove the screen. It will hold in humidity a little better.

If the light seems to be stressing the snake, you could try an infrared bulb (that's what I use) or even a ceramic heater, but those can get pricey.

Looks like a nice setup. I agree you want hides on the warm & cool ends.