Just as another approach, I've always used flat (but cut to the relative shape with some extra) pieces of fleece for the bottom "sheet" on boats. Funny thing is, I was first given this idea before heading to the tropics :hot There was one of those "boughten sets" of bedding that was V-shaped, and had Velcro and comforters built in -- but there was no way to keep it because it was too bulky and too hard to wash and dry (for the situation). Someone suggested fleece, and I thought it sounded crazy and would be hot as all get out. But counterintuitively, it always felt comfortable, even in the muggy heat. Of course it was also great in cool weather.
The thing is, it tends to slightly "grab" the berth cushion fabric beneath it, so I didn't need to make elaborately fitted or shaped sheets for the berth. Side benefit was they were easier to stow and wash, and could double as top sheets (since they were still flat).
I still now tend to use flat piece of fleece for my bottom sheet, even though I no longer "have" to. At any rate, costs nearly nothing to give it a try, since fleece is fairly cheap by the yard. Also fleece does not need to be sewn, hemmed, or cut in any fancy way. Just cut with normal scissors and you're done. No fraying or raveling. And if it's not to your taste, you've got a fleece throw
