AFS recommends mounting the ADAHRS box(es) on a bracket between the upper longerons, just forward of the tailcone bulkhead where the static ports are located next to (that is, the static port location as per Van's plans). If the turtledeck is already on, this can be a tough place to drill accurately because the upper skin curves inward above the longerons and at least for me and my old shoulders and bifocals it was an impossible contortion challenge. I mounted mine in the same location but on the middle longerons and believe it will work out well (as it has for others). But I have not flown yet, so no actual experience to share. If it turns out mine are too low (the steel rudder cables are closer in this configuration, but that's about it), I will extend the bracket to lift the boxes up a bit, simple enough.