I didn't review this thread but the 'ideal shape' has a tapering afterbody and that conflicts with being able to see into or out of the mirror.
Getting a small mirror as close to the driver's eye is probably the best you could do. These mount on the top center of the door window.
Vintage Peep Mirror | eBay
The 4" ones can have a visor (that is probably no help), a convex lens or built-in turn signal repeaters. They also come in 3". They do require a framed window, but my VW would need a slot cut in the weatherstrip channel, so none for me.