Just spent a couple of days away walking with this backpack for my P3. It's very comfy and holds the P3 rock solid.
http://www.amazon.com/Anbee®-Professional-Backpack-Rucksack-Phantom/dp/B00WE5JEBU
http://www.amazon.com/Anbee®-Professional-Backpack-Rucksack-Phantom/dp/B00WE5JEBU