I get packing paper. Aspen works too.

My rack is one of the Reptile Basic ones, so it's enclosed on 3 sides. I have 4 1/4" holes on one side of the tub to control humidity. In the summer the holes point out to reduce humidity. In the winter the holes point in to increase humidity. This keeps it just where I want it. I use those ceramic dog dishes as water bowls (probably 4") with no issues ever for humidity unless I have the holes pointed the wrong way.