PVC and weld Rivco both good. I don't have a welding machine so not willing to spend $150 buck on a flag pole. I believe mine will hold if I tighten the nut down with a locking nut. If not plan "B".
Mine does use pre-bent steel tubing in itself strong. A Swivel might be a problem that I will correct with a mod. I could have used TWO such sections on top one bottom with a cross brace preventing a swivel.