Good question.
You would need to use the XBee modules with the u.fl connection for an external antenna. You can get a short u.fl to bulkhead SMA cable and add the external 2.4 antenna directly to it or extend it.
But you are correct, the XBees with the little rubber-duck or the XBees with the built-in patch would not work within a metal enclosure....only the XBee with u.fl is appropriate in this application.
Thanks,
Landon