On our prior TT the plastic cover fit tight, but I wondered about it blowing off as I did see it happen to another TT that was on the highway ahead/next to us. Thing lifted right up and blew off, now maybe they did not have it down all the way or something since it did not draw my attention until it was going airborne as we passed them, but apparently they can come off.
2011 GMC 3500HD 4x4 Crew DRW Duramax
2012 Sierra 365SAQ
2006 Police Roadking FLHPi