Will the top of this barrel come off in such a way as to be usable as a lid? It seems to be melted/glued on to be an integral part of the barrel, so I'm guessing the only option is to saw it off.
SolarMike's idea of using a broomstick is excellent, and that worked to place my fitting. However, because I had drilled the hole through the bottom where the thickness of the plastic has significant variations, it wouldn't seal. I even bought a Uniseal, which seems to be a fine product, but it still didn't seal.
I pondered how to put the fitting in a hole drilled on the side of the barrel, hoping the plastic would be more even there, and considered how to plug the hole on the bottom.
Then an idea dawned on me which seems obvious in retrospect: flip the barrel, drill a hole in one of the caps that's already fitted and threaded. A little pipe joint compound was all that was needed on the threads, and a gasket around the fitting.