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Buy a littler box that has a 'fine' grate. The pee will be on the bottom and he can't reach or stand in it. You can buy a version with a very fine grate, that allows paper litter to be on top and not fall through. We're just talking about the quality of the grate here. If you want, you could even tinker yourself and buy a stainless steel grate that is meant for cooking and put it on top of a regular litterbox. Or from a roll.