I use a bunny resting board that is designed to get them off wire, but it works well on top of litter too. That works great for all of them other than Sophie.
Sophie tends to pee out of the box even standing in it, but she has a big butt (the box is huge, but she still misses). I use a puppy wee pad on top of a rubber backed mat to make cleaning easier. You would think a two foot long box would be big enough. It also has high sides on 3 edges, so it is the one side she goes over on occasion.