IMHO, there is no need to expend the time and effort to build a hatch for tank removal.
Epoxy coat your new tank, install it so water will not sit against it, and glass in you new deck over it with inspection hatches.
And if it ever comes time to remove the tank (in 20 years
), you can teach your great grand kids how to use a skill saw to cut a rectangle in the deck
Who knows, by then, there will problably be "laser" saws
Lloyd