Used the stone glacier one as mentioned for a long time but came to the conclusion everyone else did which is that it doesn’t stay fully secure and can be annoying as it flops around. I started then just strapping my rifle to the side of the pack and accepting it was slower to draw which is rarely an issue. Sometimes I do want a sling though and picked up a little shoulder strap retainer(sorry forgot the masks but was a small business in MT, it’s just a tiny little metal clip) that does a great job of keeping the sling on your shoulder but still find the stock of the rifle wants to rotate forward. At one time in the past I had a little thing that had a rearward facing hook which mounted to my backpack waist belt which held the stock back. If I had that again and the shoulder strap retainer I feel it would do really well and then can always just go back to strapping the rifle to the pack when I want it really secure. I do continue to use the SG rifle sling bottom piece as I like how it hold the rifle and allows we to run the rifle lower and out of the way of overhead branches.
Does anyone know of a product like I described which goes on backpack waistband as a rearward facing hook? It has like wire internal piece and was covered in some sort of fabric.