That Pelican case is great, and definitely good for heading out into the rugged wilds. I was looking for something a little lighter and with less bulk, and came up with the Think Tank Secure Pixel Pocket Rocket:
https://www.thinktankphoto.com/products/secure-pixel-pocket-rocket
Fits the expansion cards perfectly, with clear windows to see which is which. Pockets for 6, and of course if you don’t need all of them for expansion cards, you could throw in other cards that might fit like CF, SD, or MicroSD cards. Need more slots? Their Pixel Pocket Rocket has room for 8 (non-zippered), plus a zippered pouch. Need fewer? Try the Pee Wee Pixel Pocket Rocket, with 4 unzipped slots to hold expansion cards, and 3 that are SD card sized.
Here’s a photo of mine with two expansion cards in it.
And, while not waterproof, I figure it’s good enough to keep things dry from a stray splash or light drizzle.