Use a three gallon , or three one gallons. Three pounds sanitized leadfree clear marbles will offset one liter if you ever need topping up marbles. As long as your SG is above the 2/3 sugar depletion you can add juice....blueberry juice is commonly substituted in this wine.
I do not top up with water, but will use related nice wine but only after wine is fermented dry.
By not including the must in your transfer to secondary you are avoiding major gross lees development which can send off flavors into your wine, and have been known to ruin wine.
Motto: quel che sara sara