The following are some points (random thoughts) to consider when mounting a solid state AHRS in your plane.

The AHRS contains sensitive flux gate magnetometers that measure the earths magnetic field. Materials that contain iron are ubiquitous and because we cant see magnetic fields we tend to ignore their magnetic properties. Several builders have installed AHRS in "convenient" locations that ended up to be in the midst of a magnetic field that could have been drawn by Picasso. Subsequent attempts at compass calibration end in high blood pressure and muttered expletives.

ANY MATERIAL THAT IS ATTRACTED TO A MAGNET, EVEN IF IT IS NOT "MAGNETIZED",  WILL DISTORT THE LOCAL MAGNETIC FIELD .

One item that is frequently forgotten, and often regretted, is the use of ferris fasteners and components to mount the AHRS.

Mounting hardware, connector backshells, connector hardware, brackets etc. should all be non magnetic materials like stainless steel, aluminum, brass, titanium or plastic.

Wires that carry current (what wires don't) produce local magnetic fields.

Most welded steel brackets are magnetized due to the high currents that passed through them during welding.

Landing gear legs on IVs are big chunks of iron that move. After you finish the AHRS compass calibration, put the plane on jacks and observe if the heading changes when the gear retracts.

By process of elimination, the tail cone is the least "bad" magnetic environment. The passenger and baggage compartments are out of the question because there is no telling when little Johnny may elect to pack his refrigerator magnet collection for the plane ride.

How much something will effect the AHRS will depend on how big it is, how large it's native magnetic field is, and the SQUARE of the distance between the object in question and the AHRS. Distance is your friend so keep the bad stuff away by as much as practical. Tightly twist any current carrying wire with its return lead to minimize magnetic radiation. Test all the parts around the AHRS with a small magnet. If it sticks and you can swap it for a non magnetic components, do so. If you are stuck with it, take it out and have it demagnetized.

Consider all possible candidates. Many times the problem will be hiding in plane sight (NPI).

Don't expect perfection. +-10 degrees is the legal limit (FAR 23.1327). Get it within half a tick (~3 degrees) at the cardinals and call it good.  If it still bugs you, have beer. Repeat as needed until the variation becomes "acceptable".

Regards
Brent Regan