I like the concept.
That pivot joint looks like it needs some homebuilding technology to fabricate, preferably using a decently equipped workshop.
I wonder if there is some easy gizzmo out there that a person could adapt for the task at hand. Thinking low tech here.
Mebbe snip one of those wire yard sign frame holders so only one stem sticks out from the ground, and then set a bic pen body on top of it to which the flag or vane attaches? It could swivel freely in the breeze.
