It turns out cassette tapes are exactly the right size for a business card case. People of a certain age, like me, have lots of old tapes sitting around uselessly. You can give one a second life!
Step 1: Choose a Cool Tape
Start with a cool tape, like these. And no, don't be horrified that this "museum piece" Apple data tape is being destroyed--no one ever saw it when it sat in a drawer!
Step 2: Unscrew Case
First, note that some tapes are screwed together, while others are heat-welded. Obviously it's a lot easier to take apart the ones with screws. If it's heat welded, you'll have to cut it open.
Step 3: Remove Innards
After you pull out the spools of tape, there are still a few posts and things in the way. You do _not_ need to remove all of them, because there's plenty of room inside, but you'll need to remove a few.
It's both easier and cleaner to remove most of the internal parts with a pliers, twisting things until they break. You can remove everything with pliers except the one big fat screw boss in the lower center of the case.
Step 4: Tape Hinge
The easiest way to make a hinge is simply with a piece of tape. However, there's some technique to this:
First, don't use tape that splits easily, like packing tape. Your hinge will die quickly. The best tape to use is gaffer's tape--it's both thin and flexible, and it'll never get any adhesive residue on your cards. Electrical tape is ok, but it's a little stiff, and can cause the case to spread. However, with a tape case this color, you'll never get gaffer's tape to match, so clear electrical tape can be used. Or you can just trim wrong-color tape cleanly.
Second, don't just tape straight over the top wall of the cassette. The tape will want to stay straight and will push the two halves of the cassette case apart from each other. Instead, fold a little seam in the tape above the joint, and make sure the tape doesn't stick that well to the top edge of the wall. That way when the cassette case is closed, the tape will push inward and let the case close all the way.
Step 5: Optional Additions
You may decide you're done at this point. Check the fit to make sure everything's ok. Make sure the case doesn't spread open from the tape hinge when it's closed.
If the tape doesn't hold itself closed well enough from the friction of the screw bosses, you might glue in a couple (weak) magnets to act as a clasp.
If you want, you can cover up the little windows in the case so your cards aren't visible from the outside and so no dirt or gunk gets inside. I haven't found this to be necessary, personally.
Step 6: Done!
Enjoy your recycling and your retro-cool!