Finally got some private and public feedback on this:

Garmin Varia Tail Light To Fizik ICS Saddle Mount

Seems to fit well, someone attached it with a non-Fizik saddle bag and the bag was still accessible. I am asking everyone who bought the beta to send me pictures. Please note that tolerances on Garmin Quarter-turn mounts vary slightly depending on the particular Garmin device, so the Eighth-turn mount I made is on the snug side. I did not want there to be any looseness at all in the Garmin interface.

@JonnyHK, did you do anything with the Fusion 360 file I sent you? It seems I got the dimensions correct so it should print well on a well adjusted dimensionally correct printer.

I have some beta FLY6 mounts being printed, as soon as I have some feedback on those, I am thinking about making a Fizik ICS to Fly6 direct mount, It would put the Fly6 in about the same location as the Varia, and free up seat post mounting for other things. It just appears that the ICS mount was under-utilized and under-marketed by Fizik, just as the ProLogo/Topeak clip and whatever Specialized called their clip. I ride ProLogo saddles and don't think the clip is as strong as the Fizik ICS, but I will investigate it after the New Year.