Home made security alarms

wouldn't be too hard to build an alarm yourself
all you need is a closed electrical circuit with in it several limit switches and a air raid alarm
wire it up so that the limit switches are help open by the closed entrances to the garage, the side door, the overhead door, the windows (be sure to wire the switches up parallel so only one of them need to close to close the circuit

now, if someone opens the door, or window or whatever the limitswitch will close, completing the circuit and engaging the air raid siren

the only thing that would require proper consideration is how to disarm it when you want to enter the garage yourself