I'd take Cam's advice.
Having a re-sealable hand/arm sized (6" diameter) access on the top (and actually one for each baffled section) is VERY useful if you should ever get an issue with algae or other contaminants requiring a tank cleanout.
It looks like you have room, and it could easily be added in place before you add
fuel and use the tank. A hole saw or a dremel type tool could be used to cut the hole. A gasketed bolt down cover would do the trick after that.
We have two such access ports, but a 3 compartment tank so we could only do a thorough clean out on 2 compartments.
Now's the time to do this! If it's not possible be fanatically religious about filtering (or at least screening) your
fuel whenever you fill the tank.