I need to make sure I'm clear here, what I am looking for are the 20mm ammo cans, sometimes called rocket boxes by river guides (which is not necessarily 'cause they held military rockets, tho). It's what they use for their groovers. These are pretty big. Imagine something big enough to fit a toilet seat and use as portable throne. The 20mm boxes hold 100 rounds for the M61 Vulcan Gatling like on the front of Apache gunships.
The main thing is the lids have two latches and come off a lot easier than single hinge type boxes.
If you have a deep, dark seeded desire to know all about our U.S. mil-spec ammo boxes... Search on 'M-548' to find the 20mm can.