I may have posted this before but it may be inspiring to post it again.
To keep the mess contained and convenient and to control the odor I built this room under the basement stairs. It required installing risers, a wall and a door, plus some accessories. Here's what's in the room; (dratted sideways pictures)
In the Room.jpg
Raised platform because we are not getting any younger (and it saves the cats a few steps)
24 x 36 mortar pan is big and has a smooth bottom for easy excavation
Bucket with lid and liner for weekly disposal
Overhead light plus a night light
Glade plug in
Also there is a small exhaust fan that runs 24/7 to keep the odor contained. It's a computer fan connected to a dryer vent. It's good to move a little air thru the house and this is the perfect place to pull it from. (view from inside the room)
Fan and Grill.jpg
The door
Cat Room Door.jpg
And the cat's entrance
Cat Room Passage.jpg
This seemed like a lot of work when I did it 10 years ago but it's been great since and we expect another decade of use.