The trick is to fabricate a bracket that replicates an original bracket which use mirrors made to fit the Touring Car or Roadster Windshield frame. These mirrors are available at swap meets and on e-bay. Also, very nice reproduction Mirrors are available. This first photo shows an original mirror and the fabricated bracket.
This next photo shows a common hardware item, a garage door handle. This is, in fact the handle on the inside of my new garage door, used to pull the door down when the automatic unit fails. I believe you can find them at any hardware store. Cut the handle in the center, then use a tube or pipe and insert the two peices from each end and braze together. The tube should be slightly longer than the clamp on the Side Mirrow.
Very good imagination there buddy....that should work!