Your first problem is the heat lamps. They will bring down your humidity the most. Are you using digital thermometers with a probe? Your temps might actually be higher than they need to be. Chances are you don't even need the over head heat.

That being said, your UTH should be controlled by a thermoSTAT to prevent belly burns.

Second of all, the foam core board will actually absorb humidity since it is porous. Try covering 3/4ths of the screen top with a layer of aluminum foil or tape. Do not put any tape on the inside of the enclosure though. Believe me, you will be pulling it off of them and that is no picnic.

Third of all, the big tank. There is really no need to have a tank as large as a 40g. As you can see it can be hard to maintain temps and humidity. A big tank also requires a lot of security tweaks to make the snake feel safe. You will need more than two identical hides and lots of fake plants for cover. It is also a good idea to cover the back and sides with dark paper or foam core to help close off the tank a bit. I also think it looks rather nice. You could put pictures or something on the foam board. The easiest solution would be to get a smaller tank, preferably a 20g long.