Our 17lb Border Terrier pees on a faux-grass doormat, which we cut to fit in an up-turned Rubbermaid tub lid roughly 26" x 18". When she's done we dump and rinse overboard - neat! We attached the doormat to the tub lid with a couple of zip ties on one end, so it's not likely to get lost when being rinsed. The Rubbermaid lid is tough, and soft enough material that it stays in place pretty well on the cockpit deck, unlike some cheaper harder plastic lids.
She very reluctantly learned to poop on the cockpit deck, and we pick up and toss overboard if we're at sea, or deposit in the head if not. Since she considers the boat "home", she resists pooping on the deck at the beginning of every trip, but eventually gets into the routine.
