What kind of humidity level are you dealing with? i.e. is it 20% or 40%, etc. What are you using the heat the enclosure?

You may want to try using cypress mulch instead of aspen. Cypress mulch holds humidity alot better than aspen.

You could also move your water bowl to the warm side of the enclosure....this will cause the water to evaporate faster.....which will increase the humidity in the enclosure.