I don't know if aircraft compasses are any different to marine ones, but they have small adjustable compensating magnets at right angles to each other to make up for the distortion to the earth's magnetic field caused by the various lumps of iron in your vehicle. Fields caused by your electrics present their own problems. You can get second-hand air compasses without a certificate of serviceability cheaply, I think mine was about £30 about ten years ago, they said it was off a Phantom, well, they would- keep the punters happy. Try Light Aero Spares in Shebbear, or RD Aviation, you can probably find them on the internet no bother. Also, try Yorkshire Light Aircraft, Leeds-Bradford Airport, Yeadon, Yorkshire.