There's already quite a few good instructables on making your own SD card case, but I think my idea of using a 35 mm film canister has some advantages others don't have:
1. There is almost no work involved: SD cards fit inside the canister without any modifications necessary.
2. It houses 9 SD cards -- quite a lot of storage.
3. It's water and dust proof.
4. Empty and full cards don't get mixed up.
5. It seems fitting that the housing that once protected your precious film, can still do the same for your images in the digital age.
Step 1: You Need....
1. An empty 35 mm film canister (first picture). Any kind will do, although I used a Kodak one.
2. A piece of foam of 36 mm across by 24 mm high from a pill canister (second picture), or cut your own from a piece of left over foam.
3. A piece of cardboard or stiff plastic 24 mm wide by 33 mm high with two different colours on the sides (third picture).
4. And of course 9 SD cards to fill you completed case with (last picture).
Step 2: Assembly
1. Push the piece of foam to the bottom of the film canister (first and second picture).
2. Fill the canister with your SD cards (third picture).
3. Fit the coloured card so that it divides the empty SD cards and the used ones (third picture). I put my empty cards on the green side of the card, and the full ones on the red side. In use you pull out an empty card from the "green" side, and when it is filled with photos/videos, you put it back on the "red" side.
4. Push on the lid of the canister (last picture). The foam will keep the cards inside snug and prevent rattling, and make them pop out a few millimeters when you open the lid again, making it easier to grab them by the edge. As the coloured card is a millimeter or so longer than the surrounding cards, it will stand slightly proud of them, making it easy to see its colours.
And that's it!