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I have this Radio Shack Mail Guard which operates at 315 MHz. The battery powered transmitter sends out a constant signal for just over a second when exposed to light. It works great when the transmitter is only a few feet from the reciever. However with a field strength of fundamental (uV/m) 3750 to 12,500 it doesn't work from my mailbox--about 60 feet from the base. The mailbox is a thick steel multi-family type box. The signal probably resonates beautifully inside the box but not so well outside it.

Any ideas on extending the range?

Well, I'd avoid upping the power on an Air Force channel.

The instructions show it mounted on the door, so that it's unshielded while triggered. Otherwise, you might experiment with passive antennae that direct the signal... maybe snake a wire through the Faraday cage without making contact.