Need Help - Cats with Kidney Disease

Two of my old cats - 15 years - have developed kidney disease. Of course, I am following my veterinarian's advice to give them quality of life. But, one of the cats over shoots the litter box. I understand this is a problem with kidney disease.

When I lived in my house, the box was in the basement. The puddles weren't a problem to clean up. Here, in my apartment, the litter box is in a big, carpeted closet. I have "installed" a tarp to cover the carpet, but it is light weight. It pulls away from the wall and makes mopping difficult.

I have to clean up three or four times a day on my hands and knees - a difficult task for an old lady like me.

Does anyone have a suggestion to make like easier ? Some kind of waterproof pad or mat with small sides that the litter box can sit on ?

Thanks...