I appreciate that it's after the event and all that but this brilliantly simple idea could have been incorporated into production, in a slightly simplified idea at little or no cost. Just partially punch the centre hole, bend/fold the partially punched section rearwards to form the flat and there it is! Flat across the barrel and......... Where the flat sits radially is academic!

Just wonder why you put the sling loop at the front instead of a pair of holes along the top seam