For the volume system…
…wielding a backpack or any other storage item should add the item’s storage capacity.
I gained notice of how useful this could be playing some more SS13. Backpacks are handled as individual containers- open one, put stuff in and close it, you can take it anywhere and then drop it and open it again later.
Alternatively, give each storage item a drop-down list in your inventory to store items- this way having two hoodies doesn’t allow you to store a sniper rifle by cutting between the both of them. Add a new key like M that allows you to move items from storage item to storage item.
And in terms of canteens:
Make them wearable! They won’t encumber you whatsoever (-1 encumbrance on torso) and still store liquid. This would make them MUCH more valuable. However, you can only wear two at a time, and they can be easily damaged.