The other thing that occurred to me is that, if one were to top up the
carboys to 1/2" when the temperature is as warm as expected, there would be
absolutely NO need to top them up when the airspace subsequently increased
due to temperature drop. There would be no more air in the larger space. It
will simply have "de-compressed", expanded to fill the space available, just
like Boyle predicted.

I wish I had a proper cellar deep in the ground.