Articles related to commercial production are usually aimed at finding the absolute minimum ideal. It does not necessarily mean they would not be better with higher protein. This study also notably failed to look at any other variables besides crude protein.
Keep an eye on the plants they offer in the pet section of Amazon. The prices fluctuate like crazy but with a little luck and some patience you can score some great deals.
I think a porous material of some kind could work if it was machine washable. My concern with carpet tiles would be how stiff they are. You can't really throw them in the wash to sanitize them. I suppose you could try to soak and scrub them in the sink but that seems like a lot of work when...
Shelf liner is non-porous which means you can spray or rinse it off. A carpet is porous. All the liquid from their poop would soak in and become a breeding ground for disease.:sick