Fifteen envelopes, measuring 2.25″ x 3.5″, made from 28-pound brown kraft paper, with a gummed flap
After trying various options over the years, I have settled on these small paper envelopes for storing cleaned seeds in a wide range of sizes. The bottom corners are usually closely folded, preventing leaks of even tiny seeds, but I advise double-checking them before you add any seeds. The top has a gummed flap that will stick closed if you moisten it. The seal up there is not quite as tight, though, so if you are storing very small seeds, like those of petunias or mountain mints (Pycnanthemum), you may want to tape around the edges or else use a smaller glassine envelope as a liner, just for extra security.
(In the photo here, I included a few baptisia seeds as a size reference; they are not part of this item.)