Most of my bikes get a custom made alarm at some point. It consists of: 1. a hidden switch. 2. a fuse with wire. 3. 12 volt flashing led. 4. mercury switch. 5. alarm siren.
Most of the parts can be bought from a Radio Shack. The flashing LED is optional but I like to have it because I always forgot I turned on the alarm. I bought my LED off of ebay and it flashes red, blue, and green, then all different combonations of the three for a quite dazzling effect. It also came with a built in resistor which is needed when putting led's on a 12 volt battery. The mercury switch is set to turn on the siren when the bike is lifted off of the sidestand right before the bike reaches vertical. I mounted it in the flyscreen. Here are some pics.
