I use hog rings on the top and bottom. It holds the flag(s) nice and tight.
Quick question. Doesn't St. A's require an orange flag on the top only? It's kinda required out here (more of a guideline that can be enforced) and I think it's the same for Oregon as far as the dunes go. I'm not totally sure though. I know that just to be safe, I personally run a small safety orange flag on the very top and then run whatever below that.