When I had a driver side mirror installed, I wanted the guy to get a metal plate at least 2x thicker than the door metal, weld nuts to it then glue it to the inside of the door. Then screw the mirror on, which would hold the plate in case the glue failed. What he did instead was drill 3 holes in a plate that the screws would thread into and pop riveted the plate on. The rivets are hidden under the mirror base. NOT my choice. Unless somebody else has other ways, I think those are your 2 options, without finding a door that has factory attachment. Just be sure to place the mirror in the right spot if you use one of the 2 options I gave.
