I used the woodscrew ones - I think they screw into predrilled plastic blocks, or something like that, rather than tapped holes in metal. Overall length 22mm, threaded length 14mm, 8mm socket.
These look about right.
The difficult thing is making the "plates" which are fixed between the door mirrors and the door, and carry the "male" press stud fittings. Mine are made from a stiffish plastic. It needs to be thin to fit under the mirror interface. The originals are metal sheet, finished in undercoat and required painting. As you say, unobtainable, so you have to make your own. The position of the fitting, partially over the rubber of the quarter light, precludes fitting it to the door's sheet metal.
You can see the paint flaked because my plastic plate is too flexible
Something like the plastic used for printed circuit boards might be better.
Warning: The plastic shell you have to prise away to remove the mirror unit is very brittle. You can see I had to search for a replacement by the colour - I could only get a Champagne Silver one, whereas the car is Mercury Silver.
There are some helpful posts on the site - search under "tonneau". The Manuals section has the fitting instructions.