I doubt if the iron box I would have made would be air tight with the (barn) doors on the front I would have built in. There would be a small gap for air. On my ammo cans, I took off the rubber seal on the top lid which made it air tight, so there is a gap on the top for air to escape. So, I don't think I would have the pressure build up from the Lipos causing the explosion.
Wish I could keep my Lipos stored away from the house as you do. I do have a small dorm fridge in the garage I could use. May go that route now that you mentioned it.