Great, thanks for the instructable! I can honestly say this has changed my life. No more sweeping up tracked litter several times a day.

I used a pair of tall clear 57l plastic containers (the high sides contain everything but can still be seen through so my cats will actually use it!) and cut an entrance in

one end. The two boxes have lugs so when stacked they leave about 1" for the wee collection. My cats aren't very keen on pebbles under their paws so I used baked clay balls (the ones used for hydroponics). My older cat was straight in it, surprisingly no need for any transition from clumping litter to the clay balls! And there is no more tracking round the house, just a couple of clay balls to pick up if he's been a bit enthusiastic with his digging.

They are a bit dusty straight out of the bag so I give them a good rinse in a collander before tipping into the tray, to stop muddy footprints round the house.

The clay balls have become a little bit whiffy so I'm going to try soaking a batch in diluted vinegar, and will leave another batch outside.