My ring poles are constructed of closet-rod doweling amd plumbing parts. I used T's for the intersection of the cross piece and upright, and elbows for the drops. The rings are held with flat magnets. Take your largest ring to the hardware store and try out the magnets to make sure that they are strong enough. Cow stomach magnets work well if you can find the square ones.
Cylindrical ones allow the rings to turn too much.
I use metal fence posts to hold the poles up. I have rarely seen a horse crash into a ring pole although they routinely crash the saracens heads. If you are concerned about crashes, I would recommend the fiberglass electric fence posts as they are flexible and as safe as anything can be around horses. Ekkhardt of Oakenwode's drive in holes. The stub MUST be short enough to slide down into hole leaving flush top.