They cannot do it humanely with a method that requires the person to breath normally to work. If they can hold their breath it will always be inhumane because they will still be struggling and have the same impending doom and physical reaction as waterboarding.
It does not matter if the chemical properties are different when the person has a working brain and doesn't want to die. Or if it is being implemented by incompetent people who couldn't even kill him with lethal injection in 2022.