Low air flow is better than too dry. They don't need a ton of fresh air - it just kills the humidity. When I had a screen tank I covered the area between the two heat lamps with a warm moist towel (make sure the towel does not touch the heat lamps if you use them). Then I covered the entire lid with aluminum foil. I molded the foil around the lamps, but covered everything. It was not air tight by any means.

I also had to keep damp Sphagnum moss in sauce cups (like from a Chinese take out restaurant sends home) in each corner of the tank. What helped the most was installing a terra-cotta pot hide (I cut a hole in the bottom and layered it with moss too) - if you soak these pots in hot water for a few hours or until they cool to about 88-90 they absorb a ton of water and it bumps humidity as the pot dries out. I have examples of the pot in my gallery.