I just did this couple months ago. Placed a Davis instruments 15" wind vane, complete with bird spike. There wasn't one already, so it was a bit more work involved - had to tap the mast cap, play with the angles a bit. As I needed more swing room so that it didn't hit the antenna, I bought and utilized a Davis extension arm. You need to use the enclosed brackets that allow you to change the angles. Used Loctite blue for all screws.
The platform of the mast cap (Selden in mast furling main - yours might be different) extends at an angle upward (it isn't horizontally flat) like a rear spoiler, so depending on where you want the windex, will determine your mounting. Also replaced the anchor bulb with an LED one.
Here are some pics:
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