Humidity control has been an old-as-time issue with reptile keepers. If you can, I suggest switching to something that allows air to flow horizontaly (rubbermaid tubs with holes drilled in sides) rather than verticaly (Terrarium screentop). But if you need to keep the terrarium I suggest adding a moderate sized water container on the cool side, and maybe even consider adding a humid hide? Another good way to go is the use of concanut fibers, as it holds moisture very well. Mix and match these until the humidty is right or your snake seems comfortable.