Piece of 1/2-5/8 cable with loops swedged on each end and a circular disc lock holding the loops together attached to something immovable or another car and then around the frame. Not fail safe, but I think most thieves would move on to something easier.
Ways to defeat it are a cutting torch or an abrasive cutting wheel, either are noisy and will more than likely be noticed at night with the shower of sparks being thrown.
There is also a motion detector type of alarm that you can put in the car that will call your cell phone or police or any number you program into it. The dialing unit can be kept in the room, the sensing unit in the car, there is no way to stop it once it's disturbed and it's wireless and the sensor has a self contained battery in it.They're used to secure construction sites and they also have a home unit. I looked into them before and of course it's not cheap, but neither is replacing your pride and joy. Hope maybe one of these things is helpful to someone here.
Link to tattle tale alarm:
http://shoptattletale.com/