How about fitting one of these on the battery:

There's a variant of the switch which has a small fuse fitted in parallel with the knob. This means that an alarm, radio and clock is still powered if the knob is removed. But fi someone tries to crank the engine, the fuse will blow and the car will be dead. For a car that's rarely used that's probably the best solution, but it would be a bit awkward to have to open the hatch before and after every drive if it's regularly driven, so maybe it's not such a good idea for you?
Fitting a switch in the crank wire from the igntion lock is probably the best way to do it.
On this side of Oeresund, we're required to fit a three circuit immobiliser if we want theft insurance
