Litter box in garage such a good idea! One commenter noted a potential problem with the car exhaust in the garage and the cat door--whether it would be sufficient to keep it out. But we had a cat door in our old house to the garage and never had an issue.
Litterbox cabinet - SUCH A GOOD IDEA! Give the cats their own little litter box cubby room. It gives them their privacy and you can contain any mess on the outside of the litter box and easily clean it up.
Idea: Make an old dresser into a cat litter box storage area.the front of the dresser is hinged so that the box can be cleaned, but otherwise it looks like a dresser with drawers on the outside. A cat door in the side allows kitty access.