I don't see how you're not frying the snake with that 100w bulb in such a tiny space. Put the temp probe in the warm side hide and see what you get. 82-85 is good for the COOL side, and it should never go below 80 at any given time. Warm side should be 92-94 constantly.

Have you tried covering the screen top with Glad Press N' Seal, foil, saranwrap, a cut-to-fit piece of plastic? Covering the sides of the tank with a thick towel or some styrofoam insulation will help keep it insulated against the outside conditions.

How is your humidity? Probably next to nil with that 100w bulb. Covering the top with anything other than a wet towel will work wonders. You will probably still have to mist the tank, but it will help. Wet towels and washcloths harbor bacteria and as air is filtered down through that bacteria and fungus-infested wet towel that is being warmed(which equals a perfect environment for icky things), your snake is breathing that air that is contaminated. Can we say Eeww?

It may be worth your trouble to go to the dollar store or to WalMart and buy a small(16qt), locking underbed box. Just drill or use a soldering wand to put holes in each side, put your heat pad under, and you would be set to go.

Hope that helped a bit and try to get everything equalized soon. You don't want your snake getting a respiratory infection because of all of this mess.