I'd just use Pathfinder's backpacks:
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Backpack, Standard
A standard backpack is constructed of water resistant, reinforced leather and holds 2 cubic ft. or 60 lbs. of gear.
Empty Weight: 2 lb.1 Capacity: 2 cubic ft./60 lb.1
1. When made for Medium characters. Weighs one-quarter the normal amount when made for Small characters. Weighs twice the normal amount when made for Large characters. Containers carry one-quarter the normal amount when made for Small characters.
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Backpack, Masterwork
This backpack has numerous pockets for storing various items that might be needed while adventuring. Hooks are included for attaching items such as canteens, pouches, other small bags, or even a rolled-up blanket. It has padded bands that strap across the chest and the waist to distribute its weight more evenly. Like a common backpack, it can hold about 2 cubic feet of materials in its main container. When wearing a masterwork backpack, treat your Strength score as +1 higher than normal when calculating your carrying capacity.
Empty Weight: 2 lb.1 Capacity: 2 cubic ft./60 lb.1
1. When made for Medium characters. Weighs one-quarter the normal amount when made for Small characters. Weighs twice the normal amount when made for Large characters. Containers carry one-quarter the normal amount when made for Small characters.
