(first post!!!) I bought my rats a bird cage smaller than yours and have a lot of odd ball habitat. It seems it's cheaper and more fun for the rats and the owner as you can modify it as you please. I covered up wired housing (small storage shelf thingy) with used paper cups and with old socks (when they chewed up the cups) and just threw them away after a week of use. They didn't look soiled or anything I just didn't see the point of washing them constantly as they were throw aways. I don't know it's healthy or anything of the nature but it kept them warm at night, I'd say that old towels should be fine. Did you construct the hammocks yourself?
Have you considered adding levels? Did the cage come with a wire base? Mine did and I'm thinking about cutting it up and sanding the ends and adding small perches and lofts to my cage. I've tried looking for replacement parts for the rat manor for adding levels but to no avail.
