We printed on our matte finish outdoor vinyl, which gives a rich look and nice bright colors. We put a grommet in each corner, and put bungee cords around the back to hold it up. The vinyl banners were on each side of the car.
The other tricky part was not to have any metal in contact with the wood of the vehicle, since it might scratch it. We made the banners long enough to wrap all the way around the posts, plus a couple inches. That worked!