They need a flat space, without walls, to run in. They don't need a C&C cage. They need square footage that is free of obstacles so they have room to exercise.
"We guys" don't sell C&C cages. Teresa Murphy, who owns this site, and provides it for free, does. On the main page of this site, she also has instructions for how to build your own C&C cage, and for not much money, at that. So she's not doing it for profit. For years, she's been the leader in the movement to provide adequate sized cages for guinea pigs, and her standards have been endorsed by several national organizations, including the SPCA. You are perfectly free to choose to put your pigs in a less than adequate cage. What you can't do, without getting pushback here, is insist that those less than adequate cages are appropriate for guinea pigs.
Coroplast is made totally of plastic, and does not absorb water. You must have purchased something else.