Fallowbuff, I have been using a Sling Saver rifle strap holder for several years now and they work great. I have them on several packs. Here is the link: http://slingsaver.net/index.html

If you are doing a lot of climbing, you may want consider using something to secure the butt end of your rifle as well. I recently started using a Gun Commander setup I've had several backpack rifle holders, including the Kifaru Gun Bearer and the Gun Commander kit has worked best for me. But, the Sling Saver works well for most hunting situations.

Start young, hunt hard, and enjoy God's bounty.