High-sided boxes. As well as jumbo-sized boxes. If Custard is like our cats, they stand so their NOSE is in the litter box, and assume the rest of them is, too. If you have a bigger cat, the only solution is a bigger box. Our former cat who was older with arthritic hips preferred a low sided box with a pee-shield that clipped around 3 sides and angled in toward the center (but not a complete roof) while the remaining cat likes the high sided one. She jumps over one of the walls to get into the box, and exits by walking out the low side. She does it every time, too, so it's just how she uses the box. I wrote about the one we go here: https://medium.com/the-daily-cuppa/we-installed-a-second-bathroom-for-our-cats-eeebaf362710 and the specific link to the box is: https://www.petco.com/shop/en/petcostore/product/so-phresh-scatter-shield-high-back-litter-box-in-gray