How should I attach an aircraft model in a wind tunnel when testing wind gust/yaw stability?
In order to measure yaw derivatives, the airplane has to yaw when in the tunnel. If you mount it at the center of gravity, which should be reasonably close to the geometric center, it will stay in the center of the tunnel section.
In the old times, windtunnel models were mounted on strings. I expect that your model sits on a long sting. This sting sits on a rotating scale which rotates such that the mounting point does not move around when the scale is rotated. Alternatively, the scale is inside the model (see picture below). If your arrangement is different, please point this out in an edit of the question!

Internal force balance and sting (picture source)