Over 20 years ago, my wife had a gemini that used a round push lock in the cabin (passenger side). You would push the lock in which sent pin attached to a cable through the bonnet latch, so that you could not open the bonnet. The second stage was that this same mechanism would also depress a cut off switch and that way, they could not get into the engine bay to fiddle with the wires. I live in Sydney and this car still had the original metal painted sun visor and was in excellent condition and was targetted by thieves quite a few times and each time they would destroy the door and ignition barrel, but could not start the car. you could probably make up your own. Good luck.
p.s. at one point I was considering running a spark plug into the petrol tank so that when they did try to start it I could take out some marshmellows and enjoy the barbeque.
have a look at this website, it looks very similar to what I had.
http://doorlockwiz.co.cc/megatronix-...tion-interrupt