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Safariland UBL low ride is probably the way to go if you want to belt mount a holster under the waist straps of a pack.
Pair it with their QLS fork system.
It not only gives the waist strap more clearance, but also allows you to easily move the holster around to other locations if you so decide.
They even sell a QLS that mounts on Molle if your waist strap has that instead.
I would also recommend getting pic related for comfort while hiking with it.