Thinking of switching to a rack system have some questions

So I'm thinking of building a rack system for my ball pythons since I plan on getting more, and maybe breeding as a hobby. my question is how do people keep their humidity levels high in racks when they keep their snakes on aspen bedding???? I don't quite understand how they do this, since aspen is a horrible substrate for humidity. Any tips????

I use little pieces of carpet in my rack bins. I have an extra set and into the washer they go when they get soiled. Humidity is high in my bins. I just added a few holes to each bin because it was too humid.

I don't use aspen in my bins, except for my corn, since he doesn't require as high of humidity as the balls. For them I use cypress. Quite a few keepers use newspaper or butcher paper (same thing as newspaper without the ink on it, lol!). Good luck, whatever you elect to use.