So we've had a mail thief problem which seems to have stopped for now, but to prevent future issues, I like to grab the mail as soon as the postie delivers it. I work from home/I'm always here, but we cant see the mailbox from the flat (we're set back behind another flat). Ive got one of these cheap driveway alarms from ebay and have stuck the sensor in the mailbox; getting my partner to play 'postie' and testing by posting replicas of the package sizes we normally get works well, and the receiver chimes as its supposed to (which would be my queue to leap into action and swoop upon the mailbox to retrieve said mail).

Only problem is the receivers range falls short of the front door by 1.3m. Sigh lol. The mailbox is insight of the front door, but has a wooden flap for the signal to go through, so thats already some of my 'range' taken up by having to go through said flap (non-removeable). Its about 18m from the mailbox to the front door. I would like to stick the receiver on the inside of the front doors glass panel, which is still juuuust insight of the mailbox without having any added walls to go through, bar the glass panel. So I somehow need to give the receiver (yes? not boost the signal being sent by the transmitter?) a boost to get the signal another 1.3m further away and through + one glass panel. Is it possible for a novice like me to add an aerial to the receiver? Its 315Mhz if that makes a difference. I DIY everything else in the house but fall short when it comes to anything electronic and hoped someone here might have a solution?

Cheers in advance, oh helpful people :)

This is whats inside the receiver:

Edited: to make the pictures smaller, sorry about that!

  Edited again: to add the link to the ebay page for the driveway alarm.