Where to mount the dual magnetometers for the GRT Horizon HX's was a head-scratcher. We thought of installing the magnetometers in the tips of the wings under the fiberglass wingtips. You can see a photo below of us trying to see if it would fit. Plus that location was too close to the heavy lead aileron counterweight and the long, square steel tube on which the lead was mounted. Not a good idea.
We next tried putting the two magnetometers though an inspection hole inside a wing. Again, that was a very difficult location, especially trying to get them accurately aligned and leveled. We finally decided to construct a fiberglass shelf on the right side of the tail cone and mount both units there. The really tricky part was getting the shelf level and in line with the longitudinal axis of the fuselage. To do this we built an aluminum support plate for both stacked magnetometers, and then used screws and nuts to support the plate on 4 corners. That allowed precise leveling of the plate and therefore the magnetometers. That was a win!