I used one a while back on the convertible tote. Don't remember how I did it though. But do remember I did not like it because the magnets were too strong. I felt that the fabric around it would not hold up to the pressure required to pull the magnets apart.
I have also used buttens with a loop closure. Not real secure. The fasteners that I have really like have been velcro. I suppose zippers would be best but I hate working with them.