Hi,
Well a lot of people only have the section where the lamp is unsealed if they use a lamp I think.
Or you could possibly use a humid hide if you have room for 3 hides he can fit in.
But, to be honest, temps are a bit more important than humidity unless he is in shed at the moment. Especially if you are getting max temps on the warm side of low 70's f.
At those temps his immune system could be weakened leading to him catching something, and I certainly don't think he would be able to digest his food easily.
And a bad burn is a lot more serious than a stuck shed. That's just my opinion mind you as I stopped using anything but newspaper once I saw how much they actually P.
Could you use a wider water bowl? Or just try a very thin layer of the substrate - its the thickness of it that is stopping the heat getting through in all likelyhood.
You need something to control the UTH's though - as mentioned a lamp dimmer will do at a push unless your temps are very variable in the room he is in.
As mentioned before you should make sure he cannot get access to temps that will burn him but equally make sure he does get the temps he needs to survive.
**edit**
Just a thought - what are the temps in your room? Just to see how much work any heater on your tank would have to do.
**end edit**
dr del