What is your cage sitting on? If it is on a flat surface such as a table top, I imagine a shower curtain would be an acceptable replacement for coroplast. You might want to secure the cage walls somehow so they cannot be pushed over the table edge. Personally, I would be very concerned about leaks around the edges of the cage since those are the places my piggies use as the bathroom most of the time. You will probably need to protect the carpet underneath the cage to be on the safe side, at least until you find a setup that works for you. You could tie the shower curtain on the grids to make a small barrier, but the piggies might be tempted to chew on it. Using a plastic tarp is another great option since you can buy a large, single piece and cut it to the size you want, rather than having to piece together shower curtains (depending on how big your cage is). Tarps usually have holes around the edges, and using a bungee cord to attach the holes to a table or to each other underneath the table could help keep the tarp in place and secure.