Catnapper - I saw this at Petsmart the other day. I though it was a pretty neat idea and I am think if picking it up mylsef.
Perhaps this could be a solution for you....

Clevercat Top Entry Litterbox


"Cats adapt easily to the revolutionary design of this top-entry litterbox. Provides the privacy that cats require while reducing odors. Eliminates over the edge mess and built-in mat helps prevent litter tracking. Great for households with dogs! 20" L x 15" W x 15" D with 9" diameter opening. Use for up to 2 cats. One year warranty and 30-day money-back guarantee. Pan liner samples included."