I made this some months ago by customizing the code that this man here did, so this was made possible due to his efforts: https://devforum.roblox.com/t/remastered-2015-backpack-gui/1370826
Besides that, I'd like to explain how you can put this into your game:
- Insert the model into your place;
- Move the model to StarterPlayerScripts;
- Ungroup the model inside StarterPlayerScripts;
- You're ready to go.
Link to the model: https://www.roblox.com/library/9216477054/Custom-Hotbar-Arklight-Studio
IMPORTANT: There are little to no comments in the code and I'm not teaching how to customize it right now due to my limited time, so I may do a tutorial in the future, but this is NOT 100% confirmed.