I really like my plexiglass ones made from some scrap material. They're stealthy.
Mine were worth around 5% for my low speed commute. I thank the faster warm up times, it's hard to have an efficient engine in the winter that doesn't get up to temp. Thanks to all the coasting, it wouldn't even reach 170* in mild NC winters.
I cut them to match the openings, drilled some holes and used zip ties to attach them.