Max Ped makes a universal ccw holster designed to fit in the appropriate compartment on your pack. They aren't expensive at all and will accommodate most pistols.
Maxpedition Modular Universal CCW Holster - Black 3501B. 3501B. Maxpedition Holsters.
This is what I use in mine and it works but could be better.
Crossbreed also makes one.
Ohai Modular Holster | Crossbreed Holsters - Ohai Modular
No personal experience with this but they make high quality products. Either one of those options should work great and will definitely be discreet.