Mounting to gunwale

Hello, I've been here often reading what all everyone has to say and thought this would be a good place to ask this question. I am going to be making a radar arch for my boat and the arch would attach to the gunwale. I am wondering if this would be a good idea what I'm showing in the attached drawing. I would place the metal inside of the arch with fiberglass under it and on top of the gunwale. Would it be better if I were to extend the sides and ends of the metal further up the arch? I understand putting metal into fiberglass isn't that great of an idea because of expansion differences but a smaller piece shouldn't cause that much of a problem, should it? What about the underside of the gunwale. Should I use the metal plate there? I can make the base of the arch taper from 2" wide down to 3" since the gunwale gets wider further back. You can see where the mounting area is in the boat pic.

Thanks for any help or suggestions,
Eric

 

A strong solution. Long holes in the bulwark