All options do not require a medical professional to administer. It does not take someone with a doctor’s knowledge or skill to make an airproof chamber. It won’t take a doctor to set up a system to add air to the chamber. You don’t need to be a doctor to rig a way to flood the chamber with another gas and remove the oxygen. Non-doctors can wheel him in, strapped to a bed. Then the regular pre-PhD’s can operate the system. Now the scientists and engineers to design this death trap may have doctorates, but they don’t need medical licenses. Design it well enough and a chimp or small child can operate the chamber controls. You will need a medical professional to declare death, though.