Here's the perfect solution.
If you look closely, you can see that the black foam in the top lid has little holes for 9 MicroSD cards. The bottom foam has 3 holes for MicroSD cards, 1 hole for a MiniSD and 1 hole for a regular SD.
Buy two of them (they're only $1), take out the black foam from the top lid of one case, and replace the foam from the bottom lid of the other case. Now you've got one thin, slick aluminum case that holds 18 MicroSD cards very securely. You can stick the other foam back into the other case, but that case won't be as cool because it will only hold 6 MicroSD cards
But it will hold 2 MiniSD and 2 regular SD cards as well, so you can give that case to your stone-age, regular SD card-using mom or grandma.
Hell, I'm going to buy some right now before I go to sleep, since the MicroSD formfactor will be the standard for a long time.