I’ve been using plastic shower curtains with memory foam mats on them, then blankets (all from the dollar store). You really need you make sure you tuck the plastic shower curtains under the mats then cover it with the blanket so they can’t shred it. Mine only do this when there’s a pellet outside the pen they’re trying to get. Pinky used to do it when he wanted to get out but I put the litter box in the front of the pen and he stopped.
You need to learn why they’re digging to try to prevent the destruction lol. You can put things like hay mats and bendy log houses in the spots too. You could also put the crate in the pen. And make sure you’re letting them out to play enough.
Foam mats can be good for flooring, just cover them with a blanket and watch to see if they try chewing it. You can do the same with carpet. And keep in mind that some carpets can be abrasive on their feet. I would still place memory foam mats around because I love them lol. They’re so soft and squishy.
Hopefully I’ve helped a bit!