I have a glass tank with a scree top as well. I also use repti-bark. I keep my humidity up by having a large surface area water bowl. I also cover about 2/3 of the screen top with a wet towel. In the morning before work, I spray the tank with a small spray bottle and wet towel. I do the same thing before going to bed.

Another option may be to just create a humidity hide with some moss.