I built a simple coroplast litter box and it has held up very well for 3-4 months with no end in sight. The piggies did put a few chew marks into it at first, but they learned to ignore it. I'm not sure why the original poster's coroplast box isn't holding up. But don't let that discourage you from trying it.
At first I filled the box with kiln dried pine shavings. That worked well except the piggies kicked it and tracked it all over the fleece. (And it is HARD to get out of the fleece for washing!) Then I switched to pellets in the litter box and that worked much better except I found that I was frequently scooping saturated, mashed pellets out of the box because the piggies really only peed in 2 corners of the litter box.
So I decided to do something a little different than anyone else I have seen on this forum. Now I use wood pellets to fill the bulk of the litter box, but I then mix in a handful of pine shavings only in the spots where they frequently pee. The combination of the shavings and pellets absorb a lot more than just pellets before it needs to be raked, and the fleece outside the box remains pine shaving free.
Give that a try.