I've been developing this backpack system, Satchel, for Roblox for a while now and wanted to share it here for developers who might want it.
You can test the system yourself at Satchel Playground.
For developers you can read the DevForum post about Satchel and how to use it. Satchel is open-source and you can view the source code at Satchel's GitHub repository.
For me it's got to be Blackout. Basically Criminality/The Streets but more refined.