A shower curtain can make a good "subsitute" for a coroplast floor, but you'll still need something absorbant over that, and under the fleece, to catch the pee. I personally use beach towels, but someone I know uses a mattress pad, and it works well. What she uses is the one WalMart sells that is labeled "cotton-rich". If you buy a big one, you can cut it to fit, and make extras, too.
Personally, for me, carefresh is MUCH too expensive, and would be much harder to deal with as far as cleaning and changing, because of the size of my cage (nearly 20 square feet). I'd almost have to get in there with a snow plow to get it all out.
My 4, 2 were on carefresh in their previous home, and they LOVE the fleece. For a while I put carefresh up in the kitchen area, and they all walk on it funny now, so I took it out and put fleece up there too.