Tighting mirrors on a 64 dart

Does anyone know how I can tighten up the original mirrors on my 64 dart 270. There are no adjustments and they work OK until I get on the Freeway and then both of them eventually fall from the air resistance. Does anyone have a simple solution that won't damage the mirrors. They are original and the chrome is in good shape, so I don't want to mess them up. Also expensive to replace. I know that there are new after market available, but would like to keep my original mirrors. Thanks