I have a glass tank with screen lid too and I hate it! I'm buying a rack with tubs asap. A few ways you can get the humidity up that have worked for me:

1. If you have a heat pad under the tank (UTH) put the water bowl over it. It will help evaporate and increase the humidity. Also, a bigger water bowl will help.
2. you can cover the screen with a sheet or towel, something that will still let air in, but also keep humidity from escaping so fast. I usually only cover 2/3 of the top.
3. I have crumpled up newspaper my bp likes to crawl under and i spray the whole tank and paper with a spray bottle at least once, usually twice a day.

My humidity is usually around 60-65 but it fluctuates pretty bad, which is why most people are going to tubs nowadays. Hope this helps and good luck!