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.