I'd be surprised if there is a readily available holster that will mount to a vertical strap, but if you have a needle, thread, about 5" of 1" nylon strap, and an hour to kill, you could add PALS (MOLLE) webbing to your backpack strap and very easily find a MOLLE compatible holster that would fit a 345.

The only issue I'd look into would be positioning and retention. I wear pack straps pretty tightly, so a pistol on the front would either ride really high, or be canted and tucked under my arm at a kinda weird angle. A secure thumb-break would get my nod.

I have a couple mil-surp MOLLE compatible holsters that I think would be right up this alley, so again, all you'd need to do would be add 2 or 3 rows of PALS on the shoulder strap with a needle and thread and you'd be set.

OR, you could use 2-3 chicago screws (or rivets, or simply stitch the holster in place by hand). I'd personally go PALS/MOLLE, so I could easily remove and replace it, and it'd be as secure as anything out there. I'd also look at putting a spare mag pouch, or double mag pouch on the other side shoulder strap.

If you're not sure what PALS/MOLLE might be, think of the tactical vests with the rows of nylon straps running around them.

I kinda like that idea of mounting a pistol to the pack straps with MOLLE/PALS - probably gonna steal idea that for one of my backpacks.