What I've done when I wanted the top closed after it was done- Make a flap and sew onto one side, on the other side either put in a magnetic snap or velcro. You might have to work at the snap or velcro since you have the pockets inside. The other option would be an elastic & button. You could open the top seam a little and insert a pony tail elastic into the seam and resew.
Good luck. If all else fails, you can make another one. I'm always making changes to my patterns and making another one.