Actually, I think I might have finally found a solution (at least for my rats). I normally make double layered tube hammocks and to prevent my hairless from chewing through to get in between the layers I just string another piece of fleece through (attached to the hammock with safety pins).
My boys also like to tear up and hide under their fleece flooring so I just throw a few scraps (1-2 foot squares) of fabric in the cage for them to do what they want with.
So far it's worked OK, but it's only been 2 weeks.
My girls tend to get extra chew-y if they're bored. I've noticed, particularly on weekends, that if I skip two or three out-times (not days, just times, they usually get two to four a day) in a row, the hammocks and other things that can be destroyed in their cage take the heat for it.
Not saying that your rats are bored, some are just chewers, but it's a thought, because I know that's how my girls work.