anyone have any experience with fabbing a large spinner; say 16" od and 18 "
long?

I haven't done it myself but I met someone who did. He made a mold or buck
that he mounted in a lathe and fixed a piece of soft aluminum to it. After
spinning it up he used a wooden dowel to lay the spinning metal against the
form. After getting the shape he welded the hole at the nose of spinner closed
and cut out the openings for the blades. Sounds pretty simple but it took him
more than 6 tries to get one he kept.

I understand that this is essentially the technique Cessna was using in the
1980s and at that time there was one old timer who threw all the spinners for
Cessna.

John Dupre'