I use something similar to this on my boat http://www.apsltd.com/gooseneck-sliding-3-8-groove.html

If you need only the slider part, it can be found cheaper, around 15 bucks

:: Relatively new owner, and sailor. The boom on my Santana
:: 22 rotates, I imagine so the torque is less when the
:: main wants to pull at an angle other than straight up.
:: The tack pin part of gooseneck broke and now I use a
:: velcro ring that goes through the tack and all way
:: around the boom. Actually works surprisingly well, but I
:: guess it's time to do it right.
::
:: I've never worked on a rotating boom before, or any boom
:: for that matter. Any advice on where to get the right
:: parts or things to look out for?
::
:: Thanks.