Still need to fabricate the ADAHRS tray, and it needs to be further than a mount across the longerons. That's only about 17-18 inches and I'd like to raise it above that by 5-6 inches.
Several builder sites show hanging trays from the J-stringers below the turtle back. I think a raised platform above the angles and across the longerons has better maintenance potential and stability, at the sacrifice of a little more distance. Further back in the fuselage presents more installation and maintenance challenges, too.