help - how to extend this monopole?

Hi, I'm an amateur amateur. My long lost brother was a ham, and enough knowledge rubbed off on me to become dangerous, so I thought I would ask the experts here.

I would like to modify a driveway sensor to be a mailbox alarm. Why you ask? Because I can't find any offerings with sufficient range. Although my house is only 300ft from the box, it's not line-of-sight, there's a hill and a forrest of trees in between. I tried one from Radio Shack and it would sound the alarm maybe 1/3 of the time it should have. All the mailbox alarms I've seen seem to max out at 300ft. I might have seen a MURS one but it was pretty expensive.

The new driveway alarm unit I have, a Chamberlain CWA2000, transmits spread spectrum between 902 and 926MHz. The specs say that the range is 1/2 mile. The infrared sensor is housed in a plastic box and there's a single wire monopole exiting the box. It's about 3.25" long as measured from the circuit
board. I would be mounting the sensor at the back of an all-aluminum mailbox. Dozens of people in Amazon reviews say this works great as a mailbox alarm but I think most have a plastic mailbox.

several questions:

If I kept the antenna internal to my all-aluminum mailbox, how much would the signal be attenuated?

What would be the best way to extend the antenna out of the mailbox? If feasible I would like to mount a rt angle whip attached to the back of the box. I found a half-wave 916Mhz whip like this at Mouser. I could drill a hole in the mailbox back and use an bulkhead connector. The antenna has a RP-SMA connector. Is it just a matter of inserting the monopole wire into the hole in the center, or should I also ground the outer shell, circuit board, and mailbox? (Which is a lot more work.)

Any help would be appreciated.

If the antenna is inside a metal mailbox it will not work unless the door is open and that points toward the receiver. You might be able to find a small 900MHz "patch antenna" that would be made of thin circuit board and maybe less than 6" square. That could mount to the side of the mailbox that faces the receiver and you can paint to match the mailbox.

There are also very low profile 900 antennas about 5" X 5" X 3/4" thick that could mount to the bottom of the mailbox and go unnoticed. These would be omni directional and vertically polarized when mounted to the top or bottom of the mailbox. I have one but the company is out of business and they do show up on Ebay cheap.
prcguy

Drill a clearance hole in the bottom near the back of the mail box. Turn the 'alarm' upside down so the antenna hangs down thru the bottom of the mail box.