cloroplast, available at any sign store (let them know why you want it, since they don’t sell to people who want to just make their own signs) and sometimes home depot/lowes. It’s a hard, flat plastic they use for outdoor signs but can be cut to size to make a floor for small animals, if you put the corners outside the cage area they can’t get to it to chew. What I am using at the moment is vinyl carpet covers with the teeth, pressed flat onto towels, they can’t reach it to chew since it’s flat and held tight and if they dug it up it would make a loud noise, which scares them. Putting edges of that outside of a x-pen is used by some rescue fosters, which is where I got the idea (but I have no cage, so I had to improvise).
The key is to make sure they have a FLAT surface with no edges they can reach so they can’t get a grip, since buns will chew anything they can get their teeth onto.