When I was a kid I lived in a fairly high crime area, also back then cars had no technology and were much easier to steal than they are now. My mate came up with a cool idea; he wired a reed switch (magnetic switch) into the ignition and the car would not start unless he waved a magnet over a particular point on the dash. He had a colourful/humorous fridge magnet in the car which nobody would think meant anything.

Cheap and effective and not much work. If messing with the wiring is too much though (and a lot harder on a modern car) then I think you are out of luck.