Once you have the concept down, you'll need to go shopping for materials. We found the kind of storage cabinet we wanted for this project, took measurements at the store, then went hunting for a plastic container to serve as a litter tray. An inverted wire storage drawer served as an entrance/exit deck. We also needed four 1" L-brackets.
Step 1: Measure and cut ingress / egress
Step 2: Extra hardware
Step 3: The litter canopy cage
Use cutters to snip away cage wires where you've marked. Be sure to carefully file/grind any edges!
Step 4: Final assembly
Be sure the litter tray can easily slide out for scooping and cleaning.
Hmmm. Looking at this photo, I may yet add another shelf at the entrance for the cats.
Step 5: Placement
We got a cabinet that locks. This is useful because it prevents the cats from coming out the front and getting excess sand on the floor. If your cabinet doesn't lock, Baby Cabinet Locks work well.
I can't speak for your cats, but we have two that use this box. With regular scooping / cleaning of the tray, odor hasn't been a problem.
That's it! My first instructable. Let me know what you think! :)