Anyone who lives with an indoor cat knows how bad it can be when the cat uses its litter box. A cat box is unsightly to see, and worse to smell.
My first step in addressing the problem was to put the cat box out of sight in a dead space under our stairs. I cut out the drywall between studs and made a plywood box to fit behind the space (16h 14w 23d inside dims). I wanted the cat to be willing to go in and have room to turn around etc. so the whole affair is a bit larger than a standard cat box. I made a waterproof litter-box out of 1/8" plexiglass to fit in the box, be fairly high to contain a lot of litter, but not so high the cat would be discouraged from using it (8h 13 1/2w 20d). I trimmed the room side of the drywall cut-out with oak and it looked pretty good. But it was always obvious when the cat had made serious use of the box - it stunk.
The next step was to build a powered ventilator so a fan would suck air out of the box and away from the room when needed. I got some drainage tubing with the idea the fan should be placed away from the box so as to not scare the cat. I bought a 12 volt muffin fan at the electronics store. This one came with LEDs that came on when the fan was powered and I ended up using them. I got a cheap security light controller - the kind that senses motion (based on infrared heat sensing actually) and a plug adaptor that would screw into the light socket.
I cut a big hole toward the back of the box top, stapled on some wire mesh, and attached the tube with duct tape. I disassembled the motion sensor from the light sockets and mounted the sensor inside the box, wires led upward and out of the box. I made a tiny hole in the oak trim for one of the LEDs. 120v power comes from a plug in the basement, and 12 volt power comes from a power converter (wall bug) plugged into the light socket. When motion is detected, the light socket is powered, the converter makes 12 volts, the LEDs come on (one I can see in the basement and one within the oak trim), and best of all, the fan comes on. I've set the security light adjustment to medium sensitivity and maximum duration (12 minutes). 12 minutes really isn't enough to kill the stink but it sure helps.
Update from about a year later: Every time I would walk by the cat box I'd still notice the stink. 12 minutes of vent time just wasn't cutting it. So I bypassed the ever-so-clever motion detector and now the vent runs 24x7. I checked and it draws 3 watts. Well worth it because there is no stink at all in the room.