Pop rivets don't sound fun. I replaced all of the ports on my Cal this year, and used 1/4" smoked abrasion resistant acrylic, although I could have used 1/2" as well (changed from portholes to portlights and the frames had a 1/4" outside groove and 1/2" inside groove...weird). I sealed the ports into the 2-part frames using 3M Marine Adhesive 4000 UV after dryfitting the new cut pieces and using masking tape to protect the visible portion of the acrylic from the adhesive. After the adhesive set, I attached the frames to the boat with 4000 UV the same way: dryfit, mask, caulk. The ports did have a trim on the interior that attaches to the frame with ss screws, but I still went with a marine adhesive instead of silicone to give it better bonding strength.
To mitiempo's comments:
Agree, do not use Home Depot silicone, and Dow 795 is very hard to find! I tried! But Dow 795 is also most crucial as a silicone for your ports; other silicones (i.e. West Marine stuff) are fine for most applications on the boat.
If you go the bolt route, he's spot on. I chose the adhesive route. It's actually pretty easy. Just double glove and change the outer glove ANY TIME you get ANY adhesive on it. Anyone who's worked with marine adhesives can testify to their ability to transfer with the lightest of contacts. Stuff's worse than Kudzo! (You double glove because changing gloves on that sweaty hand is impossible to accomplish in less that 5 minutes per hand).
If and when you use a marine adhesive, use the 4000 UV wherever there is a chance it will show. 4000 UV will not yellow when it cures, live 4200 and 5200 will.
To be honest, I hated the whole port replacement process. Put it off until the very last project before the boat was in the water. (As in attaching the ports to the hull the night before!)