Both EFIS uses magnetometers to sense the earth magnetic field and indicate the magnetic heading.
The sensors have to be installed far enough from any steel parts. other builders have found three suitable places : wingtip, behind the baggage compartment and in front of the horizontal stab.
I choose the latter and fabricated a small bracket that will allow the magnetometers to be precisely aligned with the EFISes in the panel, which is not perpendicular to the main fuselage longeron.
All hardware used is aluminium.