Use the fermented juice (wine by this time). We usually use whatever is left after we transfer it from the primary fermentor to the secondary carboy. Put in into a container of appropriate size, airlock it, and save it for topping off later.
Do you know off hand a certain sized bottle and bung that works? I can look and try to match up a bung to a bottle myself but just thought maybe you might know. i.e... Like what size bung for a pop bottle?
These should cover all bottles and carboys - I have all of these and have not found a bottle that these would not work with.
I like to make at least a gallon extra of whatever I am making. That way between the rackings, tastings, filtering, etc - I will come out with the right amount that i was targeting.