Years ago, I got really fine government surplus instrument cases at surplus stores. They were about 30-36" square, and about the same in height. They had two clamp hinges on each of the four sides and had aluminum rims but the cases were fiberglass. They seal so tightly that there was even a screw-out bleeder valve to adjust air pressure. They were cheap and incredibly tough.
I used to carry the heavy Ascor 800 WS strobe power packs(40lbs.) in each, heavily packed in foam rubber. The cases were almost indestructible. The only down side was the heavy 'tips' I had to provide at each airport where I changed planes. Four heavy cases carrying Ascor packs. It used to cost me about $5o at every airport for SkyCap tips, in addition to extra airline insurance....in handling these 16 various cases.

Look around for some REAL surplus stores. Not the ones that carry sleeping bags, ...the ones that really carry shipping containers and industrial stuff and similar. The containers I used even had a govt. sereial number moulded into the fiberglass to confirm govt. standards.

Well worth your time, if you really want to find some Ft. Knox type shipping containers.