I re-purposed a Formica clad dense particle board case with carrying handle that originally contained a piece of test equipment, a Defibrillator Analyzer to be exact.
After removing some of the support blocks the analyzer screwed into, the ID of the box perimeter is a whisper larger than the OD of the hard plastic case my Ruger MkII 678 Target came in, and ten 100 round boxes of .22lr will fit below it. However, some of the ten boxes are displaced by 5 magazines, a couple of mag loaders, a boresnake, an aerosol can of solvent, a bottle of Kroil, some rags and a set of hemostats for the rare FTE.
The hinged lid is removable and has an inch high lip around the bottom of it. When removed and inverted, it's an excellent place to dump a box of cartridges and load up the mags without the loose rounds escaping to parts unknown.
With bulk ammo, I take the 5x20 hole plastic piece out of the slide top boxes and fill them with loose rounds, so I don't know how many rounds are in them, but I'm not likely to run out whether I'm plinking with bulk or shootin' the good stuff.
John