As the topic is
"Adding Side Fins" maybe it is worth mentioning that some boards like for example SB Go from early 2000s had optional side fins. That is why those boards have screw holes on the sides.
Unfortunately, there are several bad news for anybody considering adding these optional side fins:
1) Each year Starboard was making different side fins for each version of the board.
2) These fins are nowhere to be found
3) There is no description nor matching product numbers etc., etc.
4) Even if you make your own side fins for these boards, the screw holes were never reinforced enough to carry any higher wind load. They were meant only for learning windsurfing in light winds, for the beginner to learn going upwind. They were like "training wheels on kids bicycle".