Tom,
I pre-drilled the hole and then used a stainless steel screw, not sure of  
the size, but it wasn't too big. I taped off the screw holes before drilling.  
Somehow it manages to prevent the gel coat from cracking and splitting. I also  
used caulk on the threads to seal the hole. The screws got a good bite in the 
 fiberglass.
If I were to do it today, I'd measure the thickness of the mount and then  
make the screw 1/2 of an inch longer. They need to be big enough, but still fit  
the holes in the mount.
 
Rummy