You can do a few things aside from using a large water dish over the hot side of the enclosure. The more surface area of a water dish, the more evaporation will occur.
You can cover 90% of the lid with tinfoil, a damp towel, or plexiglass. This will help trap humidity.
The heat lamp will suck out the humidity like a sponge. Do you need the heat lamp to maintain ambient temps or is the heat pad warm enough to sustain the snake?