After some agonizing and a false start, I decided to just use 2 angle stock beams and a tray to hold the 2 units. Because of the proximity to the AP servo and the strobe unit on the RH side, I mounted it over to the left, effectively disabling the rear seat shoulder belt setup. Oh well.
I know from playing around with an electronic level that the alignment with the AHRS will be a degree or 2 off. I'll make adjustments by shiming the AHRS. We'll see.
The wiring harness from GRT defaults to the AHRS being close to the MFDs and the Magnetometer being remote. Should have spec'd it more carefully but didn't have doc until hardware shipped. No big deal because of FastStack but could have saved some by recycling more of the wiring harness.
The tray and beams.
Will rivet all in place except for brass mounting screws