Just using thin wall conduit, found in the electrical section at home depot. They don't need to support much weight. The blow mold weigh few pounds at most. The wind is more of a concern I think.

So I checked out the coupling nuts and those other ones with the ridges I posted. I also checked out rivet nuts and some of those that have a wide "collar" on one end. Most were too small with a 1/4" x 20 so I stepped up to 3/8". A 3/8" regular nut provided a tight fit, so I threaded 2 of those on a bolt. I then tapped that into the end of the conduit. I figured 2 nuts would give me better stability. I was pretty happy with using two nuts, I could have used a 3/8 coupling nut but needing to make 9 of these, I went the economy route. They are a tight fit into the conduit so I don't think I will need use any product to hold them in. They also didn't warp the conduit when tapping in. It may have put a slight "shape" into them but it isn't noticeable and they still fit in the opening of the blow mold. I will have to make the hole a little bigger for the 3/8 bolt.

I appreciate all the recommendations.
P.S. I am looking for one more of these reindeer. It is a BECO.