Awesome idea! I love seeing DIY stuff like this. I had some ideas for this as soon as I saw it:
It wouldn't take much to make this a free standing shelf, just some long, angled outrigger style feet in the front and a small one in the back. That way, you could move it if you had to and not have to unbolt it from the wall.
As for the bungee cord idea, yes that might work, but a much simpler solution comes to mind. Cut some 2x2 about 6" long and screw it to the front of the 2x4 that make the shelf so that it forms a lip to keep the car or truck from sliding off of the shelf.
My dad is a wood worker, I can't help but have some of it rub off on me. :mrgreen: