I read a tip on someone's cruising blog to use the Queen fitted sheet with the corner at the V of the berth.
To make custom sheets:
Take a conventional fitted sheet, sized a little larger than your berth area. Cut the 'boxing' off, about six inches in from the elastic.
Now you have a flat sheet and a strip of elasticized boxing. Take the flat sheet, and bring it to the berth cushion, and trace the cushion's shape onto the sheet. Cut that out, adding seam allowance (I use 1/2 inch).
Now you have a flat sheet in the shape of your berth. Now, sew the elasticized boxing you're cut off back to the shaped flat sheet. The elasticized boxing should hopefully be longer than the new resized sheet needs, just seam and cut the extra off.
Now you have a custom fitted sheet in the shape of your berth. I find that I have to label the ends of my custom sheets so my husband can make the bed without cursing too much