My diesel fuel tank does not have a fuel gauge or sender installed, well not that I can find anyway. The tank is mounted behind the engine under the cockpit floor, so access is extremely limited. At the moment I calculate fuel remaining from the hour meter, but that is not very accurate when offshore in a sea.

The filler opening is on the top with a right angle bend to the port side, photo attached to show the space available.
What would be my options for fitting a sender? Otherwise fit a sight glass at the outlet that flows back to the vent hose?