Found this effective $35 mounting base for off road visibility flags.. that allows them to fold down when not in use… will post more pics from the dunes this weekend with the additional quick release base 
A couple of tips - sub mount screws are a little bit of a pain. To avoid the screw heads scratching your rack cut a thin foam spacer template and place it between the mount and the T slot.
Amazon.com: Roof Rack Foldable Antenna Mount Kit – Compatible with Crossbars & Platforms with 16mm Hole, 180°Adjustable Bracket for Antennas, Flags, LED Lights | Easy-Install Channel Mount System : Electronics
Amazon.com: Roof Rack Foldable Antenna Mount Kit – Compatible with Crossbars & Platforms with 16mm Hole, 180°Adjustable Bracket for Antennas, Flags, LED Lights | Easy-Install Channel Mount System : Electronics
www.amazon.com
A couple of tips - sub mount screws are a little bit of a pain. To avoid the screw heads scratching your rack cut a thin foam spacer template and place it between the mount and the T slot.