Is there a worse place to play than in a large untreated flexible steel box?.
Yes if there are other boxes of the same volume welded to the other boxes. Standing wave generation heaven is that lad.
after having attempted to build a good sounding studio in a dutch canal barge and failed I feel your pain.
And I had the advantage of water damping half the top sides.
The only thing you can do is reduce your distortion to the minimum
and cut heavily with a parametric EQ at the frequency the whole place rings like a temple bell at.