Mythbusters had a Wheel of Death with spinning mirrors, right? I've also seen the thread with links to a patent about a device similar to this (which I can't find now).
Anyway, I was thinking about how a person might improve on this idea.
First, you need a wheel. A bicycle wheel should work.
Next you need reflectors. Oddly enough, bicycle makers already equip bicycle wheels with reflectors.
Now you need a place to hide it. One of those plastic cargo carriers that mounts on the roof should do the trick.
Finally you need to spin the wheel. With the wheel mounted horizontally in the cargo box, a small air duct in the front aimed at one side of the wheel should do the trick.
Now that the wheel is spinning retro-reflective reflectors (one per spoke, perpendicular to the radar beam) a shield should be placed over half the front the wheel.
My theory is that the (false) return signal from the spinning reflectors would be strong enough compared to the true signal from the car body that it would give a false reading.