Ammo boxes are cheap and air tight. I buy 5.56 7.62 .50 in them and they are great! (getting extremely hard to find full cans after the election
)
If you have an ammo box with a burning battery in it you'd better have that box on concrete.
Many years ago (decade) we had a thermal runaway on a Nicd and it literally melted its way through the box and out the bottom of the aircraft. It when through 3/8" aluminum and a steel retainer.