As a kid, I watched my father pull a single fuse from the family 4x4's fuse box whenever we left the vehicle unattented for any extended period. Does a practical, lower-tech deterent exist?
Anyways, I installed a switch up under his dash the disconnects the fuel pump +12v wire. It takes just a moment to flick the switch if you know where it is, and afterwards, the engine will crank and crank and crank and sort of sound like it wants to start at first, but never do anything. It would probably take several minutes to find it if you had to look for the switch, especially if it were at night and you were trying to steal the car. Seems like a good lower tech deterrent to me! The car has not been stolen since.
I have a buddy who had an old Ford in San Francisco. Once in a while he'd get in the car in the morning and notice that it felt .... strange. He couldn't put a finger on it. Then one day he had to get to work a little early and showed up at his car much earlier than normal. He found a guy sleeping in his drivers seat. Needless to say, both were startled and the homeless dude ran off, leaving a big bunch of keys behind in a keychain. Those were "master" keys to get into a whole slew of older vehicles.