Made a sternum strap for my #4 Duluth pack from some scraps. Just one seam to sew, two sliders and a buckle. That should make a real difference. I have used a piece of rope before as an 'ersatz'.
https://flic.kr/p/GqPM3q by https://www.flickr.com/photos/133956285@N05/
https://flic.kr/p/GqPM3q by https://www.flickr.com/photos/133956285@N05/
