Your combo of tank, screen top and lights are zapping your humidity.
By the way, and for clarification, BP's absorb heat over head as well as under (that's why radiant heat panels work well...). I would be careful about the towel idea as this creates a bacterial playground as well as not really putting a lot of humidity into the cage and taking it back out of the cage as the towel is drying.

I would be inclined to get a proper sized UTH and a thermostat to control it.
I would plexiglass or nearly completely foil over the screen.
I would insulate the sides and back of the tank.
I would put a bowl of sphagnum moss in the tank to help with humidity.
I would use aspen substrate.

I keep my cages and tubs at 55% humidity and the snakes always shed completely; I don't bump the humidity during shed.

Make sure you give your snake fresh drinking water daily to help with the dehydration issue. I remember the picture of your snake with the lateral folds of skin.... How is the little guy?