I have 3 siamese cats and want to get an automatic litterbox box(es) for them. For those of you who have used one, can you give me some recommendations about which one I should/ should NOT buy? There are so many options out there (and they are expensive!) so I want to make the right decision. NOTE: I want to be able to continue using my current litter, world's best (which is all natural, clumps, scoops, and is flushable). Some auto litterboxes require using a certain litter, but most commercial litters are full of harmful chemicals that I do not want my cat touching/ingesting.