Take a rectangular piece of fabric (what size you need), fold in half, sew side and bottom, hem the top part about half an inch, leave a small opening in that hem, slide a cord into the hem, finished, easy.If you want something more "round", sew side and bottom, then sew/cut the corners about one third of the bottom width. If you definitely want something perfect, cut a circle, measure it's circumference, use this measurement for the rectangle, sew the two pieces together, hem, etc...The good thing is you can custom make lots of gear derived from this construction afterwards.