I have this same mirror but the internal nut is not like counter sunk / recessed; So when I tighten the nut just rotates without becoming tight. So I have to first adjust then install by pushing hard into the end bar. Works good enough. Might try a split washer under internal nut.
$35 plus shipping and taxes (on their website). I bought this model for my 2019 Mariner and had to add tape to the small adjustable blocks for it to tighten in the handlebar tube which is about 17mm. I have yet to road test it, but it's a nice large mirror surface.
