There ya go!
Pick one side or the other where your shoulder strap attaches. Snip off the threads you used to sew it on. Sew a large flat button on the back of the haversack and sew a buttonhole on the strap. You will be surprised how may times you will use the button to take off the bag instead of raising it over hour head....especially in the rain when you won't wanna knock off your hat, etc.
All my shooting bags for my various muzzleloader have some apparatus like that. Some have multiple buttonholes on the length of the strap so that I can let 'em out when I wear heavier layers and can shorten them up again in warm weather. I like to wear mine up high against my ribs instead of down low on my hip. If you are wondering why, just try running with it hanging low. then hitch it up higher and try running again! It will get in less trouble up high and tight and unless you are packing bowling balls or cinderblocks in it you will not notice it as much either. Down low it keeps slapping against your leg, bouncing and flopping around, coincidentally making noise.