Duane,
We may need to be thinking beyond the basic tie down approach. If it gets too windy it is better to simply take them down and not be pre-occupied with the dumb things. They are hard to avoid when they land on the straight at 130mph.
When I was at Knott's the JBL boys used my 10' x 15' and we spent half the time holding on to the stupid thing and worrying about it. It was held down pretty good too. We saw one lift off and land on a car. I'm willing to give it a go but just a thought.
John