I am in the process of mounting dual ADAHRS in my Defiant and it's going to be a challenge. For one thing, I don't have a "tail" in which to mount them so I tried other areas, including under the rear seats, behind the instrument panel, and in the luggage area. In each case, with the ADAHRS connected to the Skyview display, I get at least 1 degree heading shift when I engage power flow through the main power bus (which runs front to back in the center console) or, in the case of the instrument panel mount, I get several degrees of change when moving the elevator. (The elevator is attached to the canard). So I'm left with the area behind the main fuel tank but in front of the center section spar. There is plenty of room but it is accessed through a VERY small hole and of course I'll have to run pitot and static lines from the middle of the canard (span=24') all the way to the back of the fuselage. Doable but tricky. Oh, and I am building a mount that is adjustable so that I can accommodate pitch and roll errors in the mount relative to stable flight.
A fellow Defiant builder has the GRT units and he installed his magnetic sensors out in the canard, away from everything. But they're smaller and don't require P&S hoses and I really don't like the idea of just gluing the ADAHRS into a primary structure where it will not be easilly accessible.
Just my two cents.
Vernon