Similar to iron chests, Iron Backpacks Mod 1.7.10, 1.7.2 allows you to make a backpack and upgrade it into 4 tiers, basic, iron, gold, and diamond. There are other upgrades you can apply to every backpack if you have an open upgrade slot.

In addition, an extensive configuration file is included with this mod, allowing you to customize nearly every aspect of the mod if you feel as if my recipes/backpack sizes/default settings/etc. does not fit into your idea of balance.