Okay, so we got a rigger who took us through a rigging crash course. I'm posting what I learned in case anyone else needs the info.
We created a very basic speed-rail "shelf" on the front of my SUV that supported our 230 lb rigger as he jumped up and down on it. The "shelf pprtion was a cheese plate which allowed us to attach the camera and anything else we wanted. Creating the cage with fittings was pretty easy -- the part I didn't know is how to attach it to the frame of the car.
The answer is: starter fittings and unibody clamps. Our rigger had a bag full of starter bolts that had one size of thread on one side and a standard thread on the other. He removed some bolts from the frame of my car (in my case, the tow hook bolts) and replaced them with the starter bolts. Now you have a set of threads onto which you put pipe starters. Check out the link below -- it's basically a solid metal cylinder that screws onto your threads on one side and slides into a speed-rail fitting on the other.
Another solution is unibody clamps. These are heavy-duty metal clamps that are tightened onto any stable metal part. On one side they have a clamp and on the other a threaded post onto which screws your pipe starter.
Once we had the cage built and attached to the frame, we used about 6 ratchet straps as safety and added stability. The result was a rock-solid camera mount that held our Red Epic stable at almost 95 mph with no micro vibration at all.
Here's the link: http://www.cinemagad...ductdetail/4376
And a good grip place should have this stuff on hand for rental along with a pipe organ (assorted lengths of speed-rail).
I still recommend getting a good rigger for your first time because you'll need to know which bolts you can replace with starter bolts (like which parts of your under-car mechanics don't move) and how to built the cage so it doesn't flex.
Hope this helps!