I have only installed a flux gate compass on a Catalina 28, and the auto-pilot I use has an internal compass.
If I recall correctly, the installation is supposed to be centered and away from magnetic interference (i.e. the engine and keel). This eliminates the low and aft areas from consideration for ideal operation. I am surprised at the installation suggestions offered, but I guess they work. I would mount the flux gate inside the head door on the bulkhead, since Hunter uses a wood compression and I have an aluminum one, neither having magnetic effects. This area is centered and about the waterline depth. On the Catalina the flux gate was installed on the forward berth bulkhead.