You can reduce yaw damping by two approaches, cut down on the vertical tail size or move the CG aft, but the CG is set by longitudinal stability. That leaves tail size.

But what I really don't know is - why you would want to reduce yaw damping? A nicely damped airplane is one that, when the control surfaces are released, stops rotating dright there on the proverberal dime. Big movable surfaces such as the big rudder on George's Tensor give maneuverability and also have high yaw dampening.