It’s box! Specifically, a stout little 9″x6″x1.375″ box with a sharp black interior, the perfect size for most RPG zines. (But not all! Measure first.)
Here’s one of mine with a copy of Crawling Under a Broken Moon inside:
To store a whole pile of zines inside a Zine Vault, I divide the heap in half and flip one half over. I can fit 10 issues of CUABM in there neatly, but fewer issues of, say, The Gongfermer’s Almanac, which is a thicker zine.
I can actually squeeze all 12 current issues, plus the bonus issue, in one box, provided it’s shelved with some other stuff to keep it closed. And that’s one of the big benefits of these little boxes: They make it possible to neatly shelve a whole bunch of zines.
Here’s the vault all closed up:
At $25 for four boxes, Zine Vaults aren’t cheap, but they’re a priced right for a niche product that’s quite well-made, and which works just as intended. I like them.
I’ve filled three of them, and just half of my fourth one is still open for business, so in all likelihood I’ll wind up ordering another pack of them before too long.
 For extra efficiency, flip over every other zine. I often store the contents of RPG boxed sets this way, as it keeps the books from developing a curve over time.