Nayla's Backpacks

Iron Backpacks deals with portable storage and item manipulation through tiered backpacks and a system of modular upgrades.  

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This mod is functional, but not actively maintained.
1.12 doesn't have all the features of 1.10, but, barring a dupe bug, should be fully functional/error free. Use an anvil to apply upgrades in 1.12.
For 1.16+ it looks like Sophisticated Backpacks has most of the features of this mod, if you want an alternative.

Note: If anyone is interested in taking over porting/maintenance I'd love to connect - join my Discord channel and come chat :)

Notable Features

• 4 tiers of backpacks, with specializations for higher-tier backpacks (more storage versus more upgrade points).
• The ability to equip the backpacks and have them visually appear on your player.
• Over 20 different modular upgrades that allow you to customize the abilities of your backpack.
  • These range from smart gathering to inventory restocking, from quickly depositing items to persistence through death, from visual indicators of fullness to compressing items on the go, and so much more. 
• Compatibility with several mods, most notably Just Enough Items, Inventory Tweaks, Ender Storage, Better Builders Wands, Not Enough Wands, TombManyGraves, and more.
• It is specifically coded to be efficient and performance-friendly.
• Highly configurable so that it can suit any playstyle.
• High quality in-game documentation.