So after some tinkering, I was able to make a Franklin Badge out of the all-class cosmetic "Clan Pride"
Here is a horrible screenshot of it:
All you need to do in order to make your very own Franklin badge is to read this tutorial:
Colored Decals: The Completed Edition
and this template:
(right click, then save as)
Be careful when you're running the script/executable. Do NOT enter a game or connect to a server.
After applying the decal tool, I just quit the game (type quit into console) and restarted my pc just to be super safe.
The script/executable has been updated, so it's much easier now.
Editing the badges from the "Flair!" cosmetic is easier as you don't need to run the script/executable.
I'll probably be working on something with that sometime.