I made one years ago the winter I spent in FL.
Will C's boat sat in the water next to the dock and would get destroyed by bird **** ... my rig was a PVC "T" that slid over where you attach the rope on the tower. Each branch of the "T" had string running down with CD's on them. They would spin/twirl in the wind and did a really good job of scaring things away.
Only picture I could find was a screen grab from an old old old video ... if I was to rebuilt it I would use fishing swivel pieces, less chance of getting tangled than the line