OK 20 long, heat mat with thermostat, 75 watt heat lamp, human heating pad.

Heat mat with thermostat on one end on the bottom glass. With a thermostat your temps should not go beyond 94 on the glass, so to insure that the entire mat is flush you can use little rubber feet stuck to the bottom of the mat (pick up at any hardware shop)

Heat lamp on top at the other end (not in the middle). Insure that the foil is good and well sealed. Create an opening that is smaller than the lamp's cover but big enough that the bulb does not touch any tape or screen. 3" diameter should be big enough.

Stretch the human heat pad across the back (but not touching the glass directly) You are just using this to create ambient warmth.

To add humidity to the tank get a heavy bowl and put a large sponge in it(one that will fill it nicely) fill the bowl with water and let the sponge soak up the water, then add more water until about 1/4 inch of water is left standing in the bowl. PLACE this directly under the heat lamp. You should notice an increase of at least 10% with this method. Make sure that this is a second water bowl -- the snake still needs fresh water to drink from another primary water bowl (without a sponge.)

give this a try and lets see what happens.