Iron Chests

Iron Chests is a mod that by and  expands the storage of traditional Minecraft chests. Larger chests can be built with new recipes, or existing chests can be enlarged using upgrades. Although the chests from this mod cannot be turned into double chests, they can be placed next to each other and therefore can be more compact. Once upgraded, they cannot be downgraded (i.e. into a lower tier chest or a traditional vanilla ).

Shulker Boxes
The Shulker Boxes added by Iron Chests were moved to a different mod,. They are still present in older versions of the mod, however.

Chest Upgrades
An existing chest can be upgraded to a higher capacity by right-clicking on the chest with the upgrade in hand.