I tried to post a photo but apparently the photo file size is too large and I haven't quite figured out how to compress or shrink or whatever the photo.
But here is what I did...
-Take off the center yoke nut...34mm socket...the fork cap.
-The inside of the nut is concave with a center pont clearly defined...self centering.
-Drill a hole for a tap. For a 1/4x20 thread, use a #7 or .2110 drill.
-Tap hole with 1/4x20 tap.
-Use permanent red threadlocker (Locktite) and from the bottom install 1/4x20 stainless steel machine screw. I don't remember the length. 1/2" seems about right.
-Install a Ram Mount (google it) 1" ball with a female 1/4x20 thread. Don't thread lock the ball unless you have a deep wall socket.
Option...
-I also used a 1" stainless steel coupling nut to get Ram mount ball a little higher.
This may or may be for you, but it works for me and some of the things I do. I also utilize the handlebar mirror pedestals for Ram mounts. I use the OEM gps mount for the 660.