I was looking for a suitable case for storing and transporting large Li-Po batteries for drones for spraying agricultural chemicals. Considering the risk of ignition of the batteries, plastic is not completely safe. On the other hand, aluminum is not strong enough. So finally, I found an iron ammunition case that originally had made for a military purpose.
When I put eight batteries, each battery is about 1.6kg and the case itself is 3.2kg, so the total weighs 16kg. But there is no distortion around the handle unlike an aluminum case I tried before, so it is quite sturdy. I need a bit strength to open and close it, but it can also be expected to be airtight to block water and air. Since there was a convex part at the bottom, I put a cushioning material to make it even.
Eight batteries just fit into the case.