I have used an Eberlestock weapon sleeve and a Kifaru Gunbearer.
However, my current favorite system is a simple sling stud attached to my pack strap.
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It's just a stud that affixes to the pack strap. It keeps the sling of your rifle in place.
Between my flexible rubber sling. Slogan Outdoors Slings. The Original Rubber Slings. As seen on TV and this sling stud, my rifle is secure (even over rough terrain) and easy to access, without adding stuff to my pack the adds weight or bulk.