I have removed the plastic bag, it made a lot of noise and was starting to inflate pretty bad (and I did a very good job at sealing it up). I have been looking around, and it is possible that my shape isn't optimal...
a cone with a low half angle has better aero than a high half angle...
My half angle was around 15 deg, and was a wedge with a half cone on the end. Cd around 0.5? Better than the stock mirrors I would guess.
but from what I have seen for Cd, a half sphere or cupped half sphere does a better job than a cone...
and an aerofoil has a much lower Cd than those!
So I may be making my mirror more "aerofoil-like" by rounding the front out and tapering the rear. I will have to see about coming by some clear plexi for cheap or learn some more about the PETG (or similar) forming.