This is a replacement for a thing stolen off my car antenna a while ago. This is a work in progress, I may change the width of the wind vanes and add a stop to the top. Basically, this slides along the antenna, from the bottom to the top as your vehicle goes faster.
I made this in 2 parts (the flyer and the flag, so that I can change out flags depending on my mood. :)

5/29/13: Added a sketchup file and included a blank flag part.

I printed these with 3 shells, 10% density, and a raft for each. The pieces are a tight fit with each other, I just tapped them together with a hammer handle and the dovetail holds it in place. I used reflective tape from an auto parts store for the red part of the flag. Have fun with it!