I have not found a satisfactory way to carry a pistol while backpacking yet. I have been using a North Face and a Kelty pack thus far and have an Eberlestock JP9 on the way. I am considering trying the "large padded accessory pouch", but quick access might be an issue.
Thigh rig maybe?
What kind of setups or holsters do you run to carry a defensive pistol while backpacking?