Firstly, I'd like to point out that I take absolutely no credit for this, it is not my tool, and I did not discover it. The only reason I am making this is because it seems like no one else has found it yet.

I also believe this is the only mod tool which allows you to edit your backpack space as of yet.

Screenshot:
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Downloads:

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Make sure you download all of the above.

Tutorial:

• Download and install Python 2.7.3 (Keep the Python Install Directory in the default location, for easy use).
  
• Download the ZIP/RAR found at the "Downloads" section of this post.
  
• Extract the 'SaveGame.sav' from your SaveXXXX.sav file. This can be done by placing the SaveXXXX.sav into Horizon and clicking 'contents'.
  
• Create a folder and place all of the contents in it (including the ZIP and Save files).
  
• Rename the 'SaveGame.sav' which was extracted from SaveXXXX.sav to something other than SaveGame.sav, E.G: 'Save.sav'.
  
• Run the application (The application will require .NET Framework 4.0).
  
• Where it says 'Data Location' navigate to the FOLDER which contains all of the contents (the application, savefile.py & your save).
  
• Where it says 'savefile.py Name' leave it as savefile.py (if you've change the file savefile.py then modify this)!
  
• Where it says 'Game Save Name' input the name of the savegame.sav you renamed (E.G: Save.sav).
  
• Modify the save how you like.
  
• Click "Mod Borderlands 2".
  
• Wait for the DOS to close and you should then see a file in the directory which contains all of the information named 'SaveGame.sav'.
  
• Replace the SaveGame.sav within the SaveXXXX.sav contents (using Horizon).
  
• Rehash and resign your save.
  
• Move the save to your USB/HDD.

All credit for this goes to Brad1571 of XboxMB.

Also, I'd appreciate it if someone could get some footage of this in action, so I can put it in to this topic, as I can't right now.