Hi, I am going to order 4 litterboxes from petfooddirect tonight when I get home from work. I was just wondering which ones I should choose from the site that would work best for my 4 cats? They tend to pee at the very bottom of the boxes, so it would be nice if the bottom had a slippery surface. I currently have 2 jumbo and 2 large litter boxes. Thanks for the suggestions.