Many years ago I work for an effects shop in Hollywood (Burbank, actually) and we needed a very heavy "car" for a commercial. We welded up a big pan on the floor (with dams that surrounded the driver's seat) and called a concrete company. It took a while to convince the driver that we weren't joking. Not as full as the one in this picture, but it added tons of weight to the car. IIRC, it was an F250 frame with the suspension removed, and a car attached to the frame. The car had to be able to drive down the street, and hit another car with its "mirror" without having the car react in any way. The mirror was 1/4" or 1/2" angle iron, welded to the frame and set in the concrete. We tore the **** out of the parked car.