A Study on the Coffee Spilling Phenomenon in the Low Impulse Regime (fermatslibrary.com)

further research: effects of salver use on low impulse regime coffee spilling

That study from 2016 recommended the overhead crab grip for carrying. There was a 2017 followup, How To Mitigate Sloshing: https://epubs.siam.org/doi/10.1137/16M1077787