Here is what I am talking about on the back. I ordered the Big Air Wakeboard mirror for around $109 on Amazon shipped. And this photo shows the backside and that ball joint.
And being that it articulated in the middle, you could pick the proper angle, no matter how you mounted it. And you could tighten the ball joint, to allow you to move it when surfing or tubing. Or tighten it down so it would not move at all.
I don't see them on Amazon anymore, but I do see them on Ebay for like $119. There is also an $89 articulating mirror on Ebay, but you need to see the backside to be sure it has the ball mount. Most have the bar going in from the side only.