This was a cheapy but goody, I found this Honsky SD card holder on Amazon, it was about $10 when I bought it at least 4 years ago, just checked and its still the same price.
My photography gear goes thru hell on my motorcycle being put to unneccessary extremes constantly and this has been in my bag for 150,000 miles or more
What I was looking for in a card holder was a secure almost crush proof case, with a secure closure that could hold both standard SD cards and micro SD cards. This holds a total of 16 cards, 8 of each.
Waterproofnees wasn't an issue because my gear is in a larger waterproof bag.
It has stood the test of time, shows a few dents, but thats about it, clasp still clicks firmly, card slots still hold cards like they did on day one.
One thing I liked was its slim, so easy to fit in a pocket if for some reason I needed to take multile cards with me, for example if I was just carrying one camera and taking A LOT of video
As I don't remember seeing it available at the time, but now the Pelican 0915 would be another option for similar storage but for about double the price, but still cheap
What are you using?