For serious backpack hunting: make a sling from thin, lightweight 1" synthetic webbing, and coat the inside with rubber cement for about 12-14 inches to keep it from slipping off the shoulder.
I use steel quick-release attachments taken from a leather sling, and lightweight lexan slide locks from REI to loop onto the attachments pieces and adjust length.
For hunting closer to a road with a heavier rifle I have a big padded leather sling!
Plan B: Put a stick in a top side pocket of the pack so that it sticks up an inch or two, and hang the rifle sling over that. I am right handed and like the stick on my left side with rifle slung forward. The rifle rides muzzle up along my left shoulder/side and I can grasp the grip, lift it off the stick and swing it forward easily. If your pack doesn't have a pocket in the right place it probably has other options to hold the stick in place. On older frame packs, the top of the frame worked. Make it a dry, lightweight stick and zip pocket or cinch straps to hold it in place.