Yes, I have the stuff to countersink and have done that for a tnut
but ran into the backing out probelm using one forward and one
backward. These would not draw down.

A fender washer has nothing on it to give a flush mount.

I would use those socket things made of plastic but I had a bad
experience with them. One time I had a set of kit speakers
and the the holes must have been drilled incorrectly and those
plastic inserts did not fit properly. So I wanted to use a
surface technique to avoid the headaches.