That was the initial plan, but you face the issue of the cars being knocked and moved around, which is a huge drawback. Not only that, de-spawning could be a problem. It's doubtful it would happen, but it's still a possibility. Not only that, but it would be kinda weird for the ball to roll over the car (esentially the goal post) and into the goal. I'm sure there would be a few disputes on whether it would count or not.
The other option, which I forgot to mention, is to use the McKenzie field hanger as a goal and take in turns to defend the goal.