Re the sugar feeding, such a small hive won't take sugar from an external feeder right now it's all just to cold for them to do that. It may also be too cold for them to venture into an internal feeder, probably best not to do that. The other ways people have suggested have merit, my own prefence is a jar with some holes punched in the lid placed upside down over the top bars, and the reason I like that is because regardless how cold it is, if the bee cluster is in contact with the jar, they will take the syrup. For a small hive like yours, to keep them warm I would recomend putting an inner cover over the top bars with a hole in the centre which the jar can sit on, and an empty super over that and then the lid. Just ensure the bee cluster is where the jar is so they can take feed without breaking cluster. Such a setup will easily keep your hive alive and prospering all winter. It does not have to be a proper inner cover, any piece of board cut to size will do.
Having said all that i am not against any of the other methods people have suggested, in beekeeping there are often several different ways to skin a cat.