Well, the first thing I might try would be to move the water bowl to the cold end of the tub so that the humidity decreases. I would also bring down the hot side temps to 92F so you don't cook your snake
Also, is the tub enclosed at all (like in a box)? If not, this might be needed to help hold in heat. Additionally, maybe you could heat a larger area????
Do you have a picture that you could post? Everything that you're doing sounds really great and now you're just tweaking things to get it perfect!!