my solution to floppy mirrors has always been to cross drill the original
post and put a roll pin in it so that the post doesn't pull out when you
tighten the nut.  you can cover the pin with black silicone if thats your
type of thing.  i have never used aftermarket mirrors but i have several
of them on hand from parts vans.  i can say that i have disassembled them
and they appear to be of lesser construction than the factory mirrors.
there is a reason that they cost less.

jonathan