Because V-berths are a funky shape, we use two flat sheets. The bottom sheet can get tucked in under the cushions all along the sides, the bottom and top, so that it lies nicely, snug and flat on the cushions. The top sheet gets tucked in at the foot and the sides. Using oversized (King) sheets provides plenty of material to get tucked in, so that it doesn’t move much, if at all. Using fitted sheets for this would be a nightmare, unless they were measured and sewn precisely to match the exact shape – which would be its own nightmare to make. Even if you had the dimensions of the berth, you’d need to know how thick the cushions were as well for fitted sheets to actually fit. A cooler might be a better idea – but let him figure out where he wants to put it.