Arf Pets Designer Enclosed Cat Litter Box
It was pretty simple to assemble, though one set of holes didn't line up perfectly so I ended up not using one of the dowels. It houses an average-sized litter box easily. I kept the door open for the first week or so while my cat got used to it, and slowly closed the door over the next few days. Now there seems to be no issue with her using it.
Easy to assemble, but having a bit of a problem finding a litter box that will fit inside.