My question would be what kind of substrate are you using? You could add some substrate that retains moisture better to what you already have in there. ie: if you use coco chip you could add some coco fiber or if you're using aspen you could add coco chip?

The other option I can think of that I use in the winter since I have gas heat is sphagnum moss. I keep some in a tub at all times for when I need to bump humidity quickly, but temporarily.

Wet the moss down and then squeeze it out so it's not soaking wet and then place some clumps in the corners or along a wall of the enclosure. It'll raise the humidity and holds it better, obviously keep something covering a bulk of the screen top so it doesn't immediately escape, and the only maintenance it needs is either to be re wet or swapped with the wet sphagnum stored in another tub.

That's the method I use for bumping humidity around shed time, because once shed is over I just pull the moss and humidity returns to normal.