renov8r,
I spent so much time looking at all the new smilies :21: that you posted before I could finish posting!
About 7' of the boards are inside the MDX and the weight and surface friction is sufficient to keep everying from moving. Also, since the sides are less than 4' wide, there is friction from the sides to hold things.
If you notice, the bungee cords on the outside go around the plywood and is mainly there to hold it down across the wood and attach to the hitch. The two inner bungee cords are used to keep the plywood from falling out. At first I didn't have the bungee cords tight enough so that the latch thingy wood was bouncing on the plywood. After I tighten everything up, it stopped rattling.
Note that the cords do NOT touch the bumper. I did this so that the paint won't rub off. I don't think I went over 50 MPH on surface street and I had no problems.
I don't think this is something I would use to go up a steep hill.
One option I considered was to build a post that inserted into the hitch receiver and extended beyond the wood and would act as a stop to keep it from coming out. There may be a bike rack that may serve the same function (and more, I think I'll look at some bike racks).
Everything fit so tightly that I didn't think it was necessary to use a post.