"Edit:  that URL was supposed to go at the end, it's the product Petco sells.    "Used this product", that is, we used a triple-layer filter in the 90s."

"Used http://www.petco.com/shop/en/petcostore/lift-n-sift-cat-litter-pan this product in the 90s, I think we bought it at Target. Can be messy and requires you to try a bunch of different litter brands to find one that works. The freshly emptied tray goes on the floor so you have to get it really clean, not just shake it off. You also now have three litter boxes to clean when you change litter, not just one.Petco already sells a sifting litter box, one with a solid floor."