Out of curiosity, I took off the OEM mirrors and compared them to aftermarket aspheric mirrors I kept from my 2009 VW Jetta. Lo-and-behold, the plastic mounting clip is identical, as are the leads for heater element. It could be my dumb luck, but some industry association may have developed a standard for the sideview mirror mounting hardware.
I've now transferred my old Jetta mirrors to my Volt. There are sizable gaps with smaller Jetta mirrors, but the increased field of view more than makes up for the "cosmetic issue", at least for me.
FYI, I flipped the right OEM mirror to see if it would fit in the left mirror housing. It will fit, but I haven't tried mounting it "all the way". That gives us one more option to use a convex mirror on the driver side, albeit with the sentence "OBJECTS IN MIRROR ARE CLOSER THAN THEY APPEAR" showing upside down