I don't use enclosed boxes for my three cats, I've never liked them - however their 3 boxes have a raised detachable top section like the one seen here
. This really helps, it makes it harder for the cats to scoop litter out. They still manage to spill a bit but not nearly as much as they did before I started using this type of litterbox. Additionally, each box sits on a large rubber mat for winter boots - the kind with a raised spill lip around the edge. This helps contain what they manage to fling about. Overall I don't worry about any spillage though, it's going to happen no matter what you do, and I just vacuum around their boxes regularly.