A little cabinet of drawers I had in the shed finally broke so I took out the drawers, painted the faux wood exterior, put in the litter box, tacked a fancy tea towel over the opening and, voila! Hidden cat box.
I'm keeping the little clip on the front for now to make it easier to enter/exit. I think I could take it down and she'd still use it but I'm not ready to test that yet. The woven rug in front is one we picked up in Costa Rica. As far as we could tell those types of rugs are absolutely ubiquitous in the South and they are pretty good at wiping litter off cat feet.