For the hoop I just used hard thick black tubing I found and it's about 1&1/2" or 2" in diameter (I think you could buy it at Lowes or Home Depot. I used a connector that I had to hook them together (so it stays tight so no water can seep in) but you can buy them at Lowes also. For the netting I bought black fiberglass screen door material (cost about $7.). I sewed the netting on with heavy fish line and made sure there was a bowl shape to it while I sewed it on. It was easy to do but time consuming. I had to throw some rocks into the net after I put it into the pond so the screen would stay down under the water. No fish can swim through the screen door material as the holes are too small except maybe tiny little baby fish when first hatched but that would be a good place for them to escape the other fish if they did go into it.
Now if the goldfish just stop jumping or sliding over into it all would be fine.