Hi Pirl, thanks for this very useful tip.
I use half inch rebar as I prefer something that goes solidly into the ground and I use it mainly for staking dahlias, and these are very often heavy, top heavy, plants. A twenty foot rebar divides into four five foot poles, two feet into the ground and three feet above will keep most plants upright, shorter for shorter plants.
I have been using cane toppers on them, but following your article I have wandered down to the local hardware store and bought a few drawer knobs and, funny enough, the rubber feet you put on wooden legs of chairs to prevent them from marking the floor.
The edge of copper tubing is as sharp as a knife, and hollow, so I would warn us all to be very careful about using it and make sure, as Pirl advises, that a topper is used at all times. I find corks split and are useful for only one season. Also modern corks are pretty poor and split horizontally.
Thanks