In the UK if you are blind in one eye you have to have additional mirrors on your vehicle. I use a left eye Safe Zone mirror and its plenty big but I can't see through it with my right eye. What I can do is what an earlier poster does and sweep the view with my left eye so it shows me a fairly panoramic view of what's behind. Still, for a situation where a rider was entirely blind in one eye I would absolutely use a bar (or frame) mounted mirror on the blind side, along with a helmet or eyeglass mirror on the good side. And practice with it until it became natural.
Anytime, everytime, you read a negative review of a helmet or eyeglass mirror you are witnessing the opinion of someone who did not, or could not, give the arrangement a chance(s). Think about a concert pianist and what it takes to do that and then think again about the eyeglass mirror you put in the desk drawer. I'm not judging you. I did it too. I'm glad I gave it a second chance.
As I understand it, the Safe Zone mirror extends fairly easy from stock, because it uses ball lock fittings available at any hardware store. As sold it sits around 4" from your eye. Moved forward a couple of inches it might more easily be seen by the other eye. I haven't tried this yet, but I think about it every now and then because I only have about 30 degrees of view through my left eye.