First the bigger the glass enclosure the harder to keep humidity level to where it should be but there is some option to try to get it up.Originally Posted by ~Kaitlin~
Misting on a regular basis
Partially covering the screen top to keep humidity in.
Changing substrate and use a substrate that retain humidity better (Cypress Mulch - Repti Bark)
Having a humid hide
Having a second water dish above the UTH
If all that does not help you still have the option to switch to a smaller enclosure
Good luck