Between the sheets
We have a local company that does basically what John did. But sheets were not sewn together. I think they only work on local boats because they insist on coming to your boat to measure the berths. They had us buy sheets of the quality and pattern we liked, then they cut to fit, with elastic all around on the bottom sheet that hugged the mattress (yes, mattress), and two elastic straps underneath, from one side to the other. I think they charged about $65 per birth (two sheets, top and bottom) for labor.Maybe there is a similiar company in your area.