In engneering terms the best place is in the area inside the stem clamps, so that the stem resists any expansion of the steerer caused by the plug.
In practice the expansive effort is so tiny and a carbon tube so strong that this is irrelevant, though in any case your top cap bolt won't allow it to go anywhere else. Be sure to grease the metal bts of the plug lightly, you'd be surpsised at how much water gets down there.