i use fleece blankets to line my cage, and made a DIY hay bag for my piggy, as hay feeders seem to get hay all over the outside of the cage due to being open on all sides. After a week of having my pig I realized he was using the hay bag as a litter box! I got him quite young so he fit right in the holes i cut for the hay to be accessible. He not only does all his pee in there but also all his poos! Rarely do i find one or two stray ones. Now i just rip up some newspaper and throw it in the bag and he continues to use it, even to the point where he wheeks in excitement when I go to empty it, which is quite simple. I just dump it into the garbage! I think he likes the seclusion the bag offers from prying eyes. If an open litter box doesn't work give this a shot, my little boar loves it, and when he gets bigger i can always just enlarge the openings or get a whole new bag going if needed. It also doesn't take up floor space on his cage really, he just pushes it out of his way whenever he feels like doing laps