The radio in my car has a USB port for iPod connectivity but also lets me use flash drives as well. I use several different Micro SD cards paired with a USB adapter so I can split my music collection up and switch out as I see fit. Initially I had this thing that looked like a business card holder and while it did its job perfectly fine, I decided it was lame. Enter the Altoids tin. Every project I've seen that involves an Altoids tin is awesome so it seemed like the perfect container for my own little project. It's very easy to do and can be done with a minimum of supplies.
Here's what you'll need:
1 Altoids smalls tin
1 sheet of 2mm EVA Foam (different types and thickness of materials can be used, but I found this the best option)
1 can of multi-purpose spray adhesive
1 X-Acto Knife (I'm using a #16 scoring blade)
Something that is about 1/3rd of an inch thick to act as a spacer
A Dremel tool (to cut the wood spacer)
At the bare minimum I suppose you could just use the foam ,the tin and a pair of scissors but the results may not look as nice.
If you want to get all fancy like I did you can also put a magnet on the bottom so you can stick to metal surfaces. For those who might be concerned, magnets will have no effect whatsoever on your data.

Step 1: Devour Your Altoids

Arguably the best part of any project involving Altoids is that you have to first empty the container. Just don't do it all at once... you'll live to regret it (or maybe not).
It's not really required but once you empty the tin, you may want to take the lid off. It won't really get in the way too much, but it is more manageable if you remove it. There are two tabs cut out of the base that are used to hold the lid in place. Just pry the tab back a bit and you should be able to remove the lid without much issue.

Step 2: Cutting the Foam

For this step you'll need a sheet of 2mm EVA Foam and something to cut it with. I used an X-Acto knife with a #16 scoring blade. You may also need a can of spray adhesive.
Most crafts stores will carry the foam sheets. You can ask for a sheet of craft foam and they should know what you're talking about. They also make foam that already has adhesive on it like a sticker, but I prefer my sheets sans glue. If for some reason you can't find adhesive backed foam and don't have any glue on hand you can just press fit them and it should hold together pretty well, but expect bowing eventually from the top layer flexing around.
You'll be taking two layers of foam and gluing them together.
The general shape is simple to get. Press the base of the tin on the foam sheet like a cookie cutter. One of the cool things about EVA foam is that if you press against it the foam will distort, leaving a perfect imprint. If you make a mistake all you have to do is leave it alone for a bit. The sheet will eventually go back to it's original shape. Cut the innermost part of the imprint and repeat. You now have two layers that should be pretty much the same.
If you cut it well, you should notice that the sheet will fit inside the base with no need for any glue. If it moves around you can glue it down if you wish,
Cutting the second layer is a bit more tricky. With the second layer you'll be making a form your media using the same technique you used to make the general shape. Just press down and cut. Micro SD cards have a small notch but you don't need to bother with it. The foam will hold the card tightly in place if cut well.
Once complete, spray one side with adhesive and glue the two layers together.

Step 3: Making a Spacer (optional)

This step is optional so you can skip it if you wish.
The reason I added a spacer was to put the cards closer to the lip of the base. I wasn't sure if it would make it harder to remove and replace the cards if I left it at the bottom so I'm playing it safe.
I used a piece of scrap wood, but anything of the proper thickness (1/3 of an inch) will suffice. You can also stack 4 layers of the 2mm foam and it will be pretty close.
At this point you may want to add a magnet as well. I cut a notch in the wood spacer and embedded a neodymium magnet . That way I can hang it off metallic objects if I want. Make sure the magnet can support the weight of the tin and the cards. Those stick on sheets will probably not be enough but any rare earth magnet will easily do the trick. I scrounged mine off a DVD drive I gutted. It is about the size of an Altoids smalls and works great. If you're concerned that magnets may destroy your data don't fret... your data will be totally safe. Any magnet that is strong enough to destroy the data on your cards is probably also strong enough to kill you. At that point a reformatted SD card probably won't be that big a deal.
Alright, with that completed let's move to the final step... assembly.

Step 4: Bringing It All Together

Alright, now that you have all your individual components completed, we're down to the home stretch.
If you decided to add a spacer go ahead and place it inside the base. You can also glue it down if it moves around too much.
Then place the foam sheet inside. Again, if it moves around you can glue it down, but I found that it holds very well as is.
If you took the lid off earlier you can go ahead and pop it back on.
With that completed, congratulations, you are now the proud owner of an Altoids tin media holder.

Step 5: Wrapping Things Up

All told this can be done fairly quickly and results in a cool little carrying case It took me about an hour to complete and that was with distractions.
Now you might be saying to yourself "Romeo, this is all fine and dandy but I want to enjoy my Altoids and store my cards at the same time... what then?" Well, first of all I'd want to know how you knew my name but then I'd present you with a simple solution. All you need to do is duplicate step two, but instead of gluing it to the base you will glue it to the lid. That way, the base is left with it's ability to store minty goodness and there is plenty of clearance on the lid for all your Micro SD storage needs. The only catch is that I don't know if the Altoids could cause damage to the cards or the foam. If anyone wants to take that leap, let me know how that turns out.
I hope that this guide was at least somewhat helpful and if it wasn't... well, sorry about that. I promise to do better next time.