Take-a-Look sells a small plastic mount, but I have never used mine---I just wedge mine between the side of the visor and the helmet shell .... but a lot of roadies seem to scoff at visors (I don't care but some might.) I get the long-arm mirror---plenty of bendable metal so I think almost any sort of mounting system could be devised and the arm bent to provide the necessary field of vision.
Duct tape fixes everything, I hear.