Backpacks.sk is a simple way to provide users with an alternative storage compartment. Usage and other things I'd like to assess will be listed down bellow. (For simpler retrieval of Skript, download here.)

DEPENDENCIES Backpacks.sk requires few plugins/addons to function. They are as listed:
Skript
skQuery


USAGE To make it easier for players to use, I've compacted Backpacks.sk as small as I could manage. There are two commands you should know about to operate Backpacks.sk.

/grant <player> <backpack size> (sk.grant) The usage is pretty self-explanatory, but if you still need help understanding it, this command allows you to get more slots in your backpack. Three options are available: default, expanded, and legacy. Default has 2 rows, expanded- 3, and legacy has 5. I do not advise going past 6 as it interferes with inventory storage. Customizing the amount of rows provided by each option will be given in the Skript itself. Another note, Backpacks.sk depends on permissions to offer different backpack sizes.

/backpack (No permission required) Another simple command. Use /backpack to open your self-managed storage compartment. Backpack data WILL NOT be lost upon name change. No feature allows administrators to edit other players' backpacks.

DEVELOPERS NOTE Like said in the dependencies section, skQuery is required to make Backpacks.sk work. Of course, there's a way to use this Skript without skQuery. In order to use without skQuery, you'll need a permissions manager like Essentials Group Manager or Permission Ex. Permissions for all backpack sizes are: backpack.default, backpack.extra, and backpack.legacy.

Additionally, I just wanted to make it known this is my first resource. Let me know if you have any suggestions or inquiries about future resources or even this one. Thank you!