I hope the analog feel of it will please the film photographers on instructables :)
Step 1: The film cartidge
I actually like the plastic film cartidge better, they are more reliable and close firmly.
As I had no use of this metal cartridge, i found it perfect for that little project.
You could of course use a standard film cartridge! Use a cheap roll if you discard the film or a roll you reuse after you developed it's film on your own ;).
I advise you open your cartridge it with a bottle opener, it's the safest way.
Step 2: Adapting the usb key to the film cartridge
You can store various files on the usb key memory, but i bet that image files will best fit ;).
I put a sticker on the cartridge and gave it as a birthday present to a photographer friend who loved it at first sight !