Spoiler
e=empty w=wool
[e,w,e]
[w,e,w]
[w,w,w]
Craft and place the backpack, then rightclick them to open them up.
punch will put it on your back, and shift-punch when empty will put it in your inventory.
the bags button in the inventory will drop the backpack you are wearing.
I would not completely trust them with valuable items, they are still a bit buggy.
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Depends: wool inventory_plus
License: WTFPL