I am not sure if you have tried this or not.. BUT you could put a piece of cardboard OR plastic on half the top of your enclosure, I am not sure if you use a tank or tub...
IF you use a tank with a screen mesh top, you can cover half (not one with or near a light...)
You can get some reptile moss and put it in a simple plastic dish, and moisten it very well. Try putting it on the hot side, and you may have to refresh the water daily..
But see if that bumps up your humidity level..
Just a thought..
Carol