First crack at installing an inspection hatch. Factory hatch is a tiny bit small for the hole it goes in but close enough for me. Hardware ordered for me by Avipro was dome-headed screws-not countersink. I called Mark and he verified they should have been countersink screws. The nutplates he provides at fairly low cost are also not countersink, but it's no problem to countersink the centre hole in the sheet metal with a 5/16 drill to accept the dimpled bottom of the hatch cover. I had to grind a bit off one of the dimple dies to allow it to dimple the nut plates. This system works well. I attached the hatch to the ring first and then will rivet the ring to the sheet metal skin. I wouldn't do this next time. I think it would be better to attach the ring to the skin first, then the hatch to the ring. It would provide greater confidence that edge distances are maintained. I haven't actually riveted the ring to the skin because it makes the hole smaller and I'm waiting to install my pitot tube, which will benefit from more access room for my arm. Not a ton of progress but had to stop and learn along the way, along with coping with an incorrect hardware spec issue. Still, I know how to do it next time, so all in all, a productive couple of days.