That looks good. You probably could make few dozens per day!  LOL  I seen those with opening in a middle for pony tail. I am not sure why that opening was needed..?. People could always put that strap above their pony tails if they would like to. WHY do you think that needs to be made with cotton yarn?? I seen those on Amazon made with 3D printers. I think each of those have 3 pairs of plastic buttons on. That way people can pick a tension they like. People do have different size