how to make up headspace when racking

I racked ~5.5g from my plastic bucket primary to a 5g carboy. It was a good thing that I had 5.5g in the primary b/c the volume of cider I was able to rack off gave just enough headroom in the carboy.

In the secondary, the cider continued to ferment and there's about 1" of accumulated lees. I'd like to rack the cider off again into another 5g carboy, but I'm going to have a shortfall and a big headspace.

What do y'all do to make up this space... boiled water? Any other options?

Some wine makers sanatize glass marbles and toss them in there to take up volume

Do you have CO2? I blow some CO2 on the top after racking.

Or, if you may want it backsweetened a little, I add a can of apple concentrate. and top off with water.