Not sure I totally understand. Is the cardboard and cardboard box not enough? Litter still gets kicked out onto the carpet?

My cat's box is in a cubby under a desk. I have a rubber-backed carpet which I put under the box and run a couple inches up the wall and side of the desk to keep the litter from getting in the crack between the carpet and side of the desk and wall. Periodically take the box out, shake the carpet outside and put it all back after thoroughly vacuuming. Works, but I only have one cat, and she's very neat.

Lucky you to have a closet to dedicate to the kitties.

I understand why the cardboard isn't enough! Cardboard gets messy fast if any of your cats misses the box.

Go to a home improvement store and see if you can find a piece of sheet vinyl as a remnant, which will be really cheap. Otherwise, they sell vinyl in 6' x 9' sizes usually (HOme Depot and Lowe's do, anyway). Then it can be cut to fit.

I put down a really nice vinyl floor in one of my bedrooms, and I can say that after years it still hasn't absorbed any smells even after older, sick cat accidents.

ooh, thank you so much for your answer.