A needle and thread to sew on some nylon webbing and install your own PALS loops would be about as straight forward as any sewing job can get. Depending on the quality of your pack, you might consider adding a reinforcing strap of nylon down the length of the shoulder strap on top while you're at it.

I bought a MOLLE compatible holster through the local mil-surp store a few weeks ago, factory model, not mil-surp, for $29. It uses 2 adjacent PALS slots and would require 3 rows. Pretty much ideal width and length for secure fixture to a backpack strap. Nearly identical to the Mil-surp versions that can be had for around $15. I paid the upcharge for the factory model to get plain coyote color.