I wrote about cleaning the tanks before, here’s a great shot that sums up installing the Seabuilt inspection port. Using the rubber gasket you pick your spot, find the center and mark all the bolt holes. Then you cut a hole in the tank and drill out all the bolt holes. Clean up the edges of any loose material and you’re done!

The piece on the right hand side folds in-half and slides into the tank. The second gasket and the solid top piece go on top and is held in place with 1/2” nuts and lock washers.