Brilliant!
Your mention of sewing supplies made me realize that I actually have the perfect storage box already--a plastic case intended to organize embroidery thread cards. The embroidery cards are a little larger than SD cards.
Here's
one at Amazon that looks like what I have. Each compartment can hold numerous SD cards, and the double compartment can hold flash drives. Probably any crafts stores that sell embroidery supplies have these, or something similar.
Thank you so much!
ETA: Just in case anyone else is looking, I found a
7-compartment version at Joann.com, on sale for only $2.09, and ordered a couple of them. I think I like the smaller version even better. Joann.com also has the 17-compartment one for $2.39. Shipping is $5.95.