Rectangular section in plan view the ‘duct’ would be vertical sided, and the top and bottom would follow the section at the aperture. When not in use, raise the motor and the bottom plate closes and fairs the aperture. Feathering not needed.

This system does not allow for steering the motor, and reversing would just be by changing gear.

A more complicated arrangement could be made with a fully rotatable thruster in a circular nozzle, but that would be a major bit of engineering, not a simple fairing on a stock outboard motor.