I installed mine on the small cross partition at the rear of that small awkward to reach little space where the stuffing box and shaft log is located. I put it fairly near to center and just under the floor of the aft berth.
Mine close to where Richard's is, under the aft bunk in the little cubby outboard of the strut mounts. Far away from the iron keel and the engine block .. just be careful with things on the bunk. The flange is facing outboard. On my old Raymarine 4000+, the autopilot had a really tough time driving until I installed a rudder position sensor.
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.
Just remember to keep your tool box stored away from the compass. I was keeping mine in the stern berth and for a while wondered why my steering compas read about 15 degress off my helm compass. Little things to remember!