I hope this can allow to clear the MiHsC question clearly, if possible with a success.
The claims are very convincing, but I cannot check if all is solid, and normally there would be some missing details to cleanup (unless Mike is lucky).
McCulloch says the microwaves bouncing around within the cavity have inertial mass (electromagnetic radiation does: that's why it can push a Solar sail) and their inertia is determined by quantised inertia. In the MiHsC theory the Unruh waves are allowed only if they fit exactly within the Hubble horizon or within a local Rindler horizon, .. if the cavity wall in this case was acting like a horizon... then the microwaves at the wide end would have more inertia than those at the narrow end since more Unruh waves would fit.as a microwave beam goes from the narrow end to the wide end it gains inertial mass. In order to still conserve momentum (mass*velocity) for the whole system, if mass goes up then velocity must go down, and the only way to achieve that is to have the whole structure move towards the narrow end.