Here is what I did for a full G3X touch system. Only item I do not have is the electronic ignition.
I had panel built by SteinAir with only a rough idea of placement. This took some head scratching but I am happy with this arrangement. Everything is sufficiently accessible with screens out.
-ADAHRS is mounted to back of left screen.
-Remote COM2 (GTR20) is on forward left part of sub-panel with connectors pointing down. This is the only box between sub-panel and firewall. I imagine you could do the same with electronic ignition.
-Rack in front left is for transponder (GTX45R). It can slide out of rack to right.
-There are no remote boxes in tail, only autopilot servo.
From left to right :
GDL51R (SXM receiver)
GAD29 (ARINC429) on center rib
GD40 CO Monitor
IBBS Backup Battery
Remote audio panel ( GMA245R ) is strapped to top of GTN650 rack.
Sub-panel cutout is for GTN650 (and connectors/wiring out back of GMA245R)