F17 port replacement

One of my F17's port rings is cracked and must be replaced. These are the round ports on top of the hull aft of the rear crossbar. I have a couple of questions related to this:

1. Can any of you F17 guys confirm that these are 4 inch ports?

2. Murrays (I couldn't find any ports in the CatSailor store) sells contoured and flat rings ( click here to see Murrays ports). Which would you recommend for this application? Or do you recommend a completely different supplier?

3. Any advice on attaching the ports? The factory ports are riveted, but I am not sure how they riveted directly into the hull. Is there a backer plate or something inside the hull that keeps the rivets from pulling through?

Thanks,
Geoff