One of the more common IKEA hacks involves converting an item of discount Swedish flatpack furniture to conceal a cat litter tray. This version uses the Expedit shelving unit, and adds its own flourish by converting the Dralla cutting board into a storage area for a dustpan and brush.
The outline for the project is posted at IKEA Hackers. As commenters note, with an entirely sealed cat toilet area, you need quality litter and regular cleaning to avoid odour problems.
EXPEDIT Cat Litter Box [IKEA Hackers]