I found these suction cup removable hooks for traveling with hanging bags. The did great attached to the glass pano roof in my back seat. They swivel so the hook actually hangs down. They held for 2½ hour trip both directions.
Amazon.com : Removable Strong Suction Cup Bathroom Wall Coat Towel Hooks Hanger 2pcs : Wall Stick On Hooks : Beauty
Amazon.com : Removable Strong Suction Cup Bathroom Wall Coat Towel Hooks Hanger 2pcs : Wall Stick On Hooks : Beauty