I would suggest a shower curtain under the towels, to protect the cardboard from moisture. Any pee will seep thru the towels and get into the cardboard otherwise, and you'll have to get new cardboard to get rid of the smell. With the vinyl shower curtain in place, you can just wipe it clean and keep the cardboard!!
My friend (lindsey's boys) just had one of the people her dad works with make her one out of what I think is metal wellded into a boxshape, then she put down the shower curtin (to make it easier to clean out) then put down the bedding.I think it works good.
I have used a large cardboard box folded to size and then buy a tarp to waterproof it, works fine with fleece on top or shavings or towels too.
My tarp was the smallest size I could buy and it cost $1.50 and I had to double it over to fit a 2by3 but it comes right up the sides just as the coroplast does.
All I use is a thick (about 1cm) piece of linoleum. Then I have a couple of layers of newspaper, then my vetbed. Even if urine soaks through to the lino, it doesn't matter because it's so easy to clean with a damp cloth.
You know how with coroplast you make a box? Well, you can't do that with lino. It's either too floppy or too thick (like mine). I don't have any sides, its just one rectangle peice of lino on the floor, with the grids around it. I did cut out sides seperately, then attatched them to the grid with zip ties. But since towels/vetbed I don't need them anymore
I started out with Lino. It's more readily available, but it can be more expensive. It is NOT durable in the long run. If your pigs nibble the side, YUK. Not good. It is harder to clean than coro. BUT , if you have to, you have to.
We just call the finished product a booklet here. Or a duotang. I've even heard them refered to as portfolios (but when I think of a portfolio I think of my huge black artbag.)