Some people will get plexiglass specially cut to cover 3/4th of the cage. If you're using a lamp, just leave enough room for the lamp to sit on the screen and the rest is covered with the plexiglass. Usually they will drill a few holes as well.
Placing a damp towel overtop works as well. Or tin foil/cling wrap. Misting helps, and so do the humid hides.
Wet substrate like Eco earth and coco fibre will help as well.
Also a large water dish that is under a lamp/over part of the heat pad can help.
What are you using right now to monitor humidity?
You may need to do two of the ideas posted to maintain a high enough humidity.