Been there - when I got my Urban Offroad rack I put it on hours before I left on a trip. I got on the highway for the first time and thought a 747 was landing on my roof!
I stopped at a wal-mart and bought a bunch of plastic for-sale yard signs, cut them up, and zip tied them to the rack. Problem solved, but pretty ghetto.
Once back from my trip I ordered the wind deflector Urban offered. Upon arrival a few days later I installed it but it wasn't as quite as my ghetto fab. So off it came and I took ti to TAP and had them duplicate it only slightly thicker (I forget what type of plastic it is - not Plexi) and about 6" longer so it would reach down to the roof. I then installed that with a strip of foam tape on the lower backside to prevent damage where it touches the paint and 1" rubber coated cable clamps to secure it to the rack. Easy-Peasy.