Good suggestions, but we are talking a LOT of velcro, something like 160" or so. I have a sleep sack and it came with all cotton sheets. I didn't like that because they are so wrinkled all the time. . . not that it matters, but I didn't like the look of the sheet. So I made a second set. I had to order the velcro off Amazon to get a roll long enough. My sheets are a microfiber wrinkle resistant. I know now why my sleep sak came with all cotton sheets. The cotton sheets don't stick to the velcro, while the wrinkle free sheets do. So those sheets are getting snagged, etc., while the cotton ones remain intact. Eventually I will make another set and use 100% cotton so they don't stick.
Dale