To maintain humidity, you can do a few tricks:
1) Cover 90% of the screen top with a damp towel, taped down tin foil, or plexiglass. (Assuming you have a tank)
2) Use a larger water dish and move it over to the hot side of the cage (wider dish = larger surface area for evaporation)
3) Make a humid hide (with a damp sponge/paper towel) instead of maintaining the whole enclosure humidity. A humid hide creates a microclimate where snake can choose to to use it if he needs some extra humidity.