Dave, I mounted my SP-4 under the passenger seat pan, as close to the center of the airplane as possible. This was the closest location I could find to the center of the 3-axis of rotation as per the install manual. Works pretty well.
The magnetometer, however, is behind the baggage compartment bulkhead...and always seems to be off by several degrees. It doesn't seem to be able to be "calibrated" out. I may create a bracket and mount it closer to the center of the airplane, since it's currently on one side.