A bit of tubing. Whatever fits over the aerial on your vehicle, which might be a different size than mine. I used 1/4 inch fuel line about four-inches long, some JB-Weld, a fishing swivel, and a three-foot long nylon streamer.
I don't drive around with the thing flapping all the time - I keep it in the glove compartment - but I slip it over the arial whenever I'm para-waiting, or parked near where I'm kiting or flying.
I always say that you can't have too many wind indicators around.
ON the topic of streamers, as wind indicators go they are said to have a unique advantage for indicating bumpy air. A European PG pilot posted that they commonly use 2.5 meter steamers made of silk ribbon, and free fly if the streamers are flowing straight and steady, but are cautious if they develop "serpentine" shapes, which indicates gusty, and bumpy air.