Do you have the lid covered in plastic and foil? This will help keep humidity inside.
Also, whenever my humidity needs a boost I put some damp sphagnum moss into each of the hides so that the snakes have the humidity where they hang out most.
Also, the heat lamp is killing your humidity. A CHE is better and none is best. Have you thought about using two UTH's (one for the warm side and one for the cool side)? Yes, you'd need another thermostat or one Herpstat II, but it would also help with the humidity issues.