I bought a used jobsite box off Craigslist for cheap from a construction business that went under. I put the ammo in GI ammo cans and the cans in the box.
The actual reason I use it, is it was "too good a deal" to pass up (don't dare laugh, most of you have bought stuff for the same reason) and it was sitting there empty. So I put the ammo in it to get that out of the way. It actually worked out quite well.
It keeps things neat, out of sight, relatively secure, safe from water (not all of it is in ammo cans), easy to get to, etc. Pretty much it's now the "Main Magazine".
I used to keep it in an old metal wall locker. That was too narrow, so you had to stack everything and invariably what you needed was on the bottom. With the jobsite box, it's spread out enough that even if you had enough ammo that you needed to stack it, there's still plenty of room to easily get at what you need to.