As for topping off...
...what's more true to the history and tradition of wine than a blend? It is totally fine to use a previous batch of wine, or a commercial version of the same type.
If you absolutely don't want to do that, and you want to be what you consider to be absolutely true to the recipe/whatever, you can:
- select a smaller carboy and discard the excess
(or freeze it for topping off subsequent batches)
- top off with water
Water will dilute the flavor, but the wine will be just fine regardless. Boil the water and let it cool, though, so that you're not dumping a whole bunch of oxygen in there along with the water, which would kinda defeat the purpose of filling up the headspace.