I worked for a local store for a number of years making the muslin covers. The shop also provided each customer with a decorative cover of their choice. It was designed to fit the full futon, both sides and had a zipper on two sides. That way you would periodically take it off and wash it as needed. If yours is to be used a guest bed, I would put on sheets for that quest. But using sheets as a cover.... you already see the problem of sheets being used as a cover. They will slip around. PM if you have additional questions.