I use the 8 inch port as it works good for storing lines and general access
for removing water. I think I generally use a 8 inch behind the splash guard
and a 6 inch in back. Between the two you can get at all the fitting bolts
except the bow handle.
My most recent inspection port purchase was about one year ago. The six-inch inspection port purchase was about $12. Protecting it from the sun was a good idea, so I spray-painted both sides before installing. The pant was a Rustoleum product—designed for plastic. A few days later, after a heavy dew, the paint started peeling off.
Oh well, it got installed anyway. In addition to whatever the slick finish was, the port seemed thin and cheap. The rubber seal couldn't seal or even fully make contact as designed. This Chinese product was no match for the old Beckson inspection ports. (Of which I have several).
If you're using screws [not bolts-not rivets] to attach the inspection port, wouldn't it be a good idea to chamfer the holes you've drilled into the Sunfish deck? (To lessen the chance of spider cracks where the deck is locally stressed).
Yes....I always wallow out the hole, usually with my Phillips screwdriver, when using self tapping screws. If not as you mention, the gelcoat can tend to crack from the holes.
Hard to describe, but these are not mounting on a deck, but inside a locker bulkhead on another daysailor for under cockpit floor ventilation.