The printed frames, M4 screws and nuts, few cm of fuel tubing and a tool on table.
I could locate one commercial option. It was over engineered with a button and springs, it turned out to be quite flimsy and broke after a few weeks of use. Thus it was not an option. I needed to design my own.
On the picture right you can see the installation going on.
The black rubbers on those screws are old fuel lines I had lying around in my garage. They are there for looks, and to provide little spring tension for the nuts. Not really a functional part, that is.
The holder will not fit Mazda MX-5 as the sun visors on that car are different in shape. But this will fit both Abarth and Fiat 124 Spider 2017 and later.
The 3D printable .stl file is in this zip file: Fiat-124-Spider-Sunglasholder.zip