My female calico used to scratch the walls by her litter box.

I got three pieces of plexiglas, one for the floor, and two for the sides. They were held together with pieces of sticky back velcro.
The litter box was in a corner of the room.

The thing that is nice about it, is that you can move them to vacuum - and if you want to move the litter box to another room for some reason, you can.

At a plastics shop they sometimes have "leftovers" and so it isn't as expensive as having pieces custom cut.