I may wrong however I am sure I am right that a ST60 and a ST60+ have the same hole size and bolt pattern, I believe the smaller hole you are thinking about is the ST50. The older ST50 wind vane
with an ST60 however the ST60 vane does work with a ST50 instrument,
You might find a dead ST60 really cheap
and then send it to Raymarine
for a rebuilt, it is a flat fee and it comes back with all of the new ST60+ guts.
It is a little hassle but you will have an as new instrument.
I have two wind vanes but no instrument,