I, personally, have been annoyed when I don't have a sleeve for my NES cartridge's. So I have made a replica for the original sleeve that usually comes with any new game like Super Mario Bros.
Step 1: Materials/Tools
Here are the materials you will need to make this:
Glue(I used Elmer's. It dries very fast.)
Cardboard(Make sure it is very thin.)
Tape(optional,if you don't want to hold it forever.)
Box Cutter(Helps in the future,trust me)