Measure everything in truly objective units: the Universe system. Define all fundamental values from the properties of the entire universe itself.

Mass: One UM (Universe Mass): mass of the entire observable universe.

Length: One UL (Universe Length): diameter of the entire observable universe.

Time: One UT (Universe Time): the time from the start of the universe to the statistically likely moment when the last black hole decays.

Thus, the best unit to use to measure change in volume would be a volume unit derived from the length unit. Or, cubic UL's.