On the face of it, this backpack mod isn't too overpowered. In fact I love this mod. But it's only when I get to the endgame in a world with shulkerboxes when I think "Maybe this is a little bit OP". Because you can put shulkerboxes in a large backpack, then put that backpack in a shulkerbox, then that in a large backpack and it is basically infinite storage in one item slot.

To fix this issue, maybe you could make it how you can still put shulker boxes in backpacks but maybe not the other way round.
Besides, I love you mod, thank you for your time.