I see that many newer Sunfish have a horn cleat on the starboard side of the mast. My older rig does not have one. Is that something I need to add?? If so, what is the best way for me to attach it to the mast? Aluminum Pop Rivets? Screws?
I used screws, and like JohnCT, put it as high up off the deck as possible. I used the 3M 4200 sealant behind the cleat, and fastened it on with #10 stainless steel sheet metal screws – I had to pre-drill the mast with small pilot holes.
this picture is a good diagram of where to put it, as well as other rigging information.