That hole looks big enough for a cigarette lighter socket! At least they put it up high where it could actually illuminate the tub if it was for a light! My DT300i has a light buried in the bottom side of the tub! I added a LED light near the top above the original. Works good. Always check to make sure the "Met-in" warning light on dash is out after closing seat!
There are so many LED fixtures available at boat dealers and stores. That's where I got my add-on light. Bass Pro marine, West Marine supply or maybeveven Amazon! Cigarette lighter sockets, too! With rubber caps!