Also, you are going to have to put something around the bottom of the panels, those gaps are large enough for them to wiggle out. I would offer more than one litter box, in a house, it's usually one more than the number of cats. If you use any type of tub, you'll need to make sure it doesn't get rained on, or it will get water logged.
You might be able to just build a type of sandbox litter box in the corner, with filter fabric under it, dug down to a certain depth, filled with sand, that you could scoop the solids from and hosing the sand down to dilute the urine.

Caroline
~I don't suffer from madness. I enjoy every minute of it.~