This is a simple set up for a stinky problem. (it works better then it looks, i just quickly set it up to see if it works)
the motion switch I used turns on for 5 minutes but can be set for 10 or 30.
I used the following:
1 cardboard box big enough for the litter box and for the cats to move in
1 small hair drier (2 dollars from a second hand store, you can use a vacuum but it make a lot more noise)
1 motion activated power switch (35 dollars)
small extension cord
laundry drier sheet
Step 1: hair drier set up
use a hair drier with a cool setting, tape down button for the cool setting and switch on low
fold laundry drier sheet and cover the end that blows, use rubber band to hold in place.
plug in to see if the hair drier is working on cool and air is flowing through the drier sheet
(i had to remove the hair drier handle so it would fit better)