FWIW--I did strap mine down using steel straps from Lowes that are made for 3" PVC. I attached 2 of them like I showed in the picture and then another 2 on each elbow. When complete, I pulled hard on it from every angle and it moved maybe 1/4".

I give up for this year, but I might revisit this project for next year. My season is almost over and I'm back to work...