I used a bicycle bar end mirror, removed the part that inserted into the bar and drilled it so it could be mounted to a rivnut in the upper forward portion of the door/window frame. Fully adjustable so I can observe the rear seat PAX or my wingman, I also mounted a circular convex mirror centered on the rear of the front spar carry thru to check rudder movement and clear above and aft while maneuvering.
If I have a spare mirror in the hangar I will post details but since it was 10 years ago I doubt that it will be available.
I think it was a Blackburn mirror, it's oval/oblong like the current ones I see on line, I shopped and compared at length and am very happy with the result.
Correction it's the Zefal Cyclop mirror which is still available, remove the bar end insert and screw into an rivnut, I used a structural washer under the screw to spread the load, mine has 800 hrs of trouble free service.