I was able to grab the flats with my hand , but there is not enough room inside to clear the length of the wrench. I like your idea of a hatch . It looks like if you cut away a 8x3” port right above the white painted wood that covers the tank, you would have access to the flats and could put a regular open end wrench on it from the inside of the boat .Loren makes a good point on the wrench flats. Adjustable wrench won’t cut it! It seems strange that there would not be a way to easily access the wrench flats and aluminum cylinders. Is there a way to create an access panel that would be hidden by the cushion?
I Also found a 50.00 endoscope that you can connect to your iPhone . ( it has good ratings on amazon) . I’m going to see if I can use it to see where the threads screw into the aluminum .