This is the Oh S..t Kit, or the "I have old Game Boy cartridge cases lying around, I think they can hold things" Kit.
My wife and I went on a two month backpacking trip around Europe, and having to live out of our bags was going to be a challenge. We were thinking about ways to compress all of our stuff without sacrificing items, and we were left with a few odds and ends that we thought would be helpful, they just had no place to go. Enter my Nintendo addiction.
Step 1: Find a Game Boy cartridge case.
You can find these for silly cheap on eBay if you must, or you can just go rough up some kids at a garage sale and get them for even less. Ask at your local EB or GameStop too, most places that take take original Game Boy games in on trade (if they still do, that is) don't want the cases.
Alternatively, you can make smaller versions of this using a Game Boy Advance cartridge case, but that is too small for most things I'm putting in here.