How To Deter Pet Rats From Chewing Cage?

We spent a large sum of money to upgrade my 4 male fancy rats (2 furry dumbos, 2 hairless top ears) to the Midwest Critter Nation since it's bigger for them and easier to clean for us. As some of you know, it has plastic slide out trays that are used for the base of each level (there are 4 levels). For the first few months we had no chewing problems at all, but recently they have chewed up the bottom pan pretty horribly on the sides and corners. I have given them other things to chew on since we got the cage, but they don't seem interested in chewing those things. They only like to chew the cage itself. They do have stuff in their cage for enrichment like running wheels, hammocks, hanging tubes, cozy cuddle beds, hanging toys, etc. but perhaps they are bored with those so they resort to chewing the cage itself? What can I give them to chew on that they will like to chew on more than the cage pans so I can stop further damage to the cage? The replacement pans are kinda' pricey and hard to find, and I'd rather avoid having to replace them by giving them something more chew friendly. Are blocks of wood safe? What about those pumice blocks I see at pet shops, or whatever they are? Are other plastic items safe for them to chew on, since they are wanting to chew on the plastic pans anyways? Please note that they are not eating the plastic, just chewing it into small shavings and getting it all over. Many thanks to any one who has some helpful advice. =}