Bigger Backpack mod for 7 Days to Die Alpha 18 game version. 60 and 96 slot versions, both with or without food and water bar on the HUD depending on what you prefer. Both have different unlocked/locked slots so pack mule will still work to unlock everything.
60 slot: 40 unlocked/20 locked.
96 slot: 61 unlocked/25 locked.
Make a Mods folder in your 7 Days to Die directory and unzip the file into there (should be KHA60-BBM/KHA96-BBM or some variation thereof for the folder name.)
The forum topic of the mod is here.