On my super-large backpack (Dana Designs Terraplane X, one of the old ones from before Marmot bought them), I have this add-on pouch that extends from one shoulder strap to the other across my stomach. It threads on to the cinch strap at the bottom of the shoulder strap on one side and has a clip at the other end. That clip clips to a strap that connects to the other shoulder strap cinch strap. It has a water bottle holder and enough room for most handguns. It would be perfect, I think. Never carried one in it, as all of my backpacking with it was as a Boy Scout on Boy Scout trips (not as an adult leader, either). I put trail food, gps, flashlight, camera etc. in it.
You might look at something like this.