fasteners for inspection port

I would recommend against using pop rivets to install inspection ports for the simple reason that if you ever need to replace the port you'll need to drill them out. I'm not a big fan of the 3M 4200/5200 stuff either. I have never used the stuff but the impression I get is that it bonds incredibly well. We are talking about an inspection port here. There is no stress so the 3M stuff seems overkill; a wood backing is overkill too. Certainly there are many ways to install one. I've done several and my method is to use stainless steel screws and 100% silicon gel for a water-tight seal. There are either six or eight screws holding the inspection port in place plus the silicon helps a little bit - it's not going anywhere.

I concure with you totally, sorry Beldar. Pop rivets might work for ports with flat bottom screw pockets as they are designed for flat surfaces for the head to seat against. A countersunk pocket puts a lot of stress along the rim of a Pop Rivet head and many times it deforms if it is an aluminum head, a steel or stainless steel head against the plastic will often crack the plastic as it pulls down the flange to contact the deck (also deforming the port flange which may afect the sealing ability of the port cover or make it hard to install or remove). The expanding part underneath (or in if you forgot the back up washers) the fibreglass puts a lot of stress on the fiberglass that may cause the gelcoat to crack in that area. Plus that little piece of broken mandril in the center is just plain ugly.

With screws, washers and nuts, you can tighten them just enough to be snug and seal (with a little sealant such as 3M Mairne Grade Silicon) without putting stress on the fiberglass deck or the port flange. A neat port installation with all the screw slots lined up just looks a whole lot better.
I'm with Brain and SuperCub with the exception that now I use one of the newer urathane caulks rather than silicon. The urathanes last longer, don't discolor or "dirty" over time, and stick better to most any surface.
The Poly Sulfide caulks like 5200 are overkill.
I came up on a sandbar and had to pull the centerboard and rudder up to sail away from it. As I reached back to push the rudder down I put some weight on the inspection port and it gave way, breaking around the inside edge and falling inside. I had to sail very carefully home so as not to take on water. Obviously I need to replace the port, is there one that is strongest of those sold? This is a Viking marine six inch port. It really seemed to push through quite easily. The outside ring is still intact so it should be an easy replacement, but I don't want this to happen again! Thanks in advance, Bob

Glad you made it home ok. How old was the port? If your SF has been stored outside deck up and uncovered, the sun may have made the plastic in the port weak. I have Beckson 6" ports and Viking 5" "catbag" ports in both of my SF and have not had any problems with either. Sea Dog, I think, is another brand that I have seen, but have no experience with.
I used #8 x 1/2" SS screws on my ports, along with Sika-Flex goop. I don't know if it's theoretically the best way to have done it, but since they've been in place for fifteen years and don't leak, I'm assuming it's an acceptable method.