12 Volt Alarm

I have been looking around for a 12 volt alarm that can be used within a lockable trailer and in the back of a sealed ute canopy (like most people, I tend to wander away from my vehicle for a large part of the day whilst prospecting). I am wondering whether anyone is using an alarm and/or how difficult are they to make (it would probably need to have some form of simple sensing device just in case someone breaks and enters through a canopy window/trailer wall panel). Any help would be much appreciated.

I bought several car alarms (I have many cars) from JB Hifi some time ago. I bought the first one and liked it soooo much, I bought several more. I'm not too sure even what brand it is, but it wasn't one of the expensive famous brands. My point though is, you should be able to go to JB or any of the auto shops and talk to the salesman there and they should be able to steer you in the right direction. I couldn't see it as too difficult to have a car alarm (with a remote control) and have one of the sensors going to the rear use door, although the canopy, if its a canvas one may takea little more in the thinking department. Maybe the saleen have tackled this problem before??
Cheers