Had this thought randomly so I went with it, what happened was I was in the GECK doing something else and saw the slave backpack in the list
but couldn't recall what it looked like so I did and well it seemed like an alright heavy duty pack so I made it playable and tweaked the stats.
This uses the base game slave backpack so you don't need any DLC or other files, just the esm.
If you have another mod that effects this item then you may encounter issues.
This mod changes the "Slave Backpack" to be playable and changes the name to simply backpack, it has 2 DT and weighs 30 lbs but gives 80
to offset it's own weight and give you 50 more max weight, it counts as heavy armour so it'll slow you down and factor into sneaking.
It has no physical form in the normal game world so if you drop it then it's gone forever!
The ID is NVSlaveBackpack in case you wanted to add it using the console command input though it never worked for me with NV, can't get it open.
You'll also be able to get it off of slaves that have it since it's now playable it'll be in their inventory when they're dead.
The main the reason I'm sharing this on here is because I wanted a backpack mod that didn't need any additional files, plug and play as it were.
Companions should be able to use it just fine as well since it's a normal piece of armour, it does use the backpack slot though so if you equip
a weapon like the minigun or flamer it'll replace it visually though it'll switch right back when you switch to another weapon type.