I'm sure there are usable off the shelf options, but on my tank I found the best places for access were oddly shaped and so I made my own. Easy enough to do. Get the proper thickness marine aluminum
from a place like online metals, and just size it over between 1/2 and 1 inch on all sides. You can use gasket
material, or a diesel
proof form a gasket
, and space bolts every couple inches around the perimeter. I tapped mine into the tank, so the heads are in the tank and the threads sticking up so all you have to do is put a nut on the bolt and tighten it down.
I had to do this as part of a full on tank clean out on Aeolus. Here are photos of my tank process and the access hatches I used. It has worked great, with no problems after encountering some seriously rough 50 knot wind
seas here in the Salish Sea. Stories of Aeolus- Our Gulf 32 Pilothouse: Fuel tank repaired in epic fashion