You are bring back memories. When I would make mine, I would use King sized sheets. Sew the top sheet to the bottom sheet, but NOT all the way across the bottom. I used to leave between 5 to 8 inches open on each end (just like the store bought sheets). I also used to add elastic straps on the under side of each corner (this helps hold them on as the water moves around). Also remember to place the top sheet facing down when you sew the two sheets together. Good luck