Inert gas, if you can get it. I'm not sure about the legalities of Halon these days, but ordinary CO2 would work, and in compressed cylinders you can store quite a lot of it in a small space. Given that we're assuming there's burning plastic you'd hope any firefighters are using respirators already, so just flood the area with CO2, deprive the fire of oxygen and there you go. CO2 is heavier than air so would naturally fill the ship, though whether you could avoid getting oxygen to the other levels I don't know. Maybe put a big tarp over the top of the ship? :P.