Tex, yes I made up a curved plate out of 14g steel to follow the contour of the hinge. I drilled and tapped the hinge 3/16 UNC or BSW and fitted the plate to the hinge.
When drilling the hinge take care not to drill through the hinge pin.....you are drilling a blind hole. I believe the hinge is a forging so it's a tough piece of steel !
The arm is made of 5/16" diameter bar and I introduced a gentle curve in to that. The mirror head is "off the shelf" stainless steel. That I painted black.
A fiddly job but it looks like it was there from day one; particularly painted in body colour