Sam, it is only Chotzetz if it is an Ohel or a Tzamid Passil. It’s not an Ohel so I guess you are considering it a Tzamid Passil. If it works for the airplane it works for the bag.
We don’t rely on this Hetter when it comes to Tevilas Keilim. Also, Chazal always use the term Barzel, which suggests that it is a rule for metal. If there is a difference it would be regarding aluminum and other earth metals, being that they are a different type of metal than those mentioned in the Torah.
The question then becomes, what about alloys? If iron is mixed in, to what extent does it matter? Do we go after Ikkar and Tafel, or Rov?