I'm not at home at the moment, but I think I have a set of "Starbilizer" side fins.
Does your 2004 GO 150 have 3 holes (inserts actually) in the rails or 4 holes?
I can check when I arrive home next week and give you patterns.... etc.
Others have sucessfully built their own side fins.
Another option would be to have a deep Tuttle fin box placed in the center of the
board, but this is non-reversible and would require either very good repair skills on your part or taking the board to a really good repair shop.
Hope this helps,