Well this is the one I would suggest.
http://www.autotechs.com/items/item231.htm
It is smaller than the normal car alarm and more importantly, it is water tight. The big benefit with is system is that it only has on it what you need so therefore it is smaller. Plus, the siren is 1/2 the size of a car siren but I swear it is 2x louder. You can also add a pager or prox. sensor if you want.
