This sounds like a difficult task.  The distance alone seems problematic.

I agree the metal container is going to be a challenge. I dropped a 2 inch wire "antenna" out of the bottom of my metal mailbox.  I am not an antenna expert, but it seemed to help.  My mailbox is only 20 feet away though.

Your only hope might be that a device would be able to transmit when the mailbox is opened.  In my experience the entire front of the wall panel is opened when mail is delivered to those large building mailboxes.  Hopefully, this would provide enough time to send out a message.