I did one inspection port with the self drilling screws, the other with nuts & bolts. Two of the self-drilling screws wouldn't completely tighten. Other than that, it worked ok.
The nuts & bolts method was more expensive, but also seems to be more solid.
I caulked with M3 4200, as shown below.
I noticed that due to the curvature of the deck, I had some gaps, so I decided to caulk once more around the outside of the the ports.
What I meant was to apply to a flexible inspection port (made in China). The better brands (Beckson, Py-Hi) would still be served by building a small platform with 4200 and squish the port on top of it. (Trimming off the excess as it oozes out).
My three catamarans (four in life-ownership) had at least four inspection ports—each, and I had to make a tool to turn the covers; which fortunately, I didn't need to access far from shore.
They're fiddly, but bolts work especially well. Since you've done both what do you recommend?