When I had my C-18 I used one of the rectangle milk crates....I made a cover of 1/2" plywood with some scrapes cut and attached to the underside to keep it in place. I stained it and varnished it and applied several strips if non-skid tape. These fit perfectly between the quarter berths. I made two and kept them onboard. The other I made the same, except I cut two holes for cup holders. They also fit perfectly in the cockpit. So you can use them as a cockpit table. When making long crossings, I would have one in the cabin, and the other in the cockpit at the companionway with a cushion. Set the autopilot and sit there watching the world go by. They also greatly increase the storage space on the C-18, and fit under the companionway when you don't need them. Some folks install the aluminum steps, this idea servers the same purpose, plus gives you storage. Also, requires no holes to drill. Here are some pictures of the first one I made. I made two others and installed 3/4 round molding on the edges. It gave a nicer finished look, and helped to keep things from rolling off when using it as a table.
Don