Anything with a screen is going to kill your humidity. Cover as much of the screen as well as possible. Keep adding water bowls. I know in my PVC enclosure, during the winter months when my furnace is on, I have to keep two large water bowls in the enclosure to keep humidity where I like it. I've even added three at times. My BP has never even attempted to "go for a soak" so don't expect it to lay in water. From the research I have done, only a VERY small percentage seem to enjoy laying in water if they don't have mites.

I'd add more water bowls, and maybe add some wet paper towels to your hides during shed time.