House Panther in the Jungle

This is my handsome house panther, Ninja, who has a problem. He is behaving like he's living freely in the jungle.

Every so often he does not use the litter box. He chooses instead any receptacle made of plastic, such as an empty laundry basket, though there are plenty litter boxes about the house.

He has also occasionally peed on the guest bed, and a couple times on the carpet. I had to replace the mattress. It was so unsalvageable. The carpet, I used a product that removes the stain and odor.

I hope this doesn't turn out to be a medical issue. ManCats do have problems that way. It can be quite painful for them, and quite expensive at the vet's to treat it. If it's a behavior problem, I am still trying to figure it out.

Could be because his first two years were spent in a van with a homeless family. As he grew up, he got to have more freedom and often went outdoors to play. Perhaps he finds being confined to the house too stressful. Or maybe I'm not cleaning out the litter box often enough. Or, maybe it's the brand of litter I'm using. Or just plain stress?

I don't know how I can work with this, aside from letting him be an outside cat. My ex-feral, Sunny-boy is very attached to Ninja and would freak out if they had to be separated. I don't dare let Sunny be an outside cat. He is way too skittish. He would be miserable. My plan right now is to get another large litter box and add it to the ones I already have. My litter boxes are actually large plastic file boxes from the business supply store. They are much deeper than a regular litter box and offers a bit more privacy and less tracking of litter outside of the box.