This design certainly has evolved - a dozen drafts of how to arrange the components on cardboard, 3 ways to fix components to the aircraft, 4 versions of how to make the removable panel. One of the very last changes came from Ross Farnum at SDS. Seems it takes 6" of clearance for the cables going into the EM-5 electronic ignition modules. I did a last minute juggle to create a clear field for the cables. No, a right angle DB25 won't work. I think all is final, and I'm drilling for the fasteners. On the fastener map, the green dots are click bond studs, a 10-32 thread and the pink X's are 6-32 rivet nuts. Both use 3/16ths holes in the aluminum, drilled and deburred. I want to prevent turning of stud or rivet nut. The click bond studs were notched and the notch aligned with a 1/16th hole drilled in the aluminum, then set with JB Weld, taped firm against the aluminum during cure. The holes for the rivet nuts are lightly notched before the squeeze, aiming to reduce risk of turning. Back to the fastener map - the purple hex shapes are the through bolts that use existing firewall penetrations. I will put the head of the bolt on the engine side and secure from the cabin side, with AN hardware and thread locker. Contactor basics and wire size. Contactors are powered switches that control high amperage flows. The battery contactor draws 0.7a continuous, with power flowing from the battery terminal, to a post of the magnet wiring, around the magnet, out through the second post and then through the battery master switch to ground. The starter contactor draws 3.5a intermittent, flowing from the battery to the starter button to the starter contactor, then out through the contactor case to the battery ground. For ease of recognition I used red and black twisted pair, in 16 AWG. Over the 14' run from panel to firewall the 20 AWG is rated for 15a intermittent, 7a continuous.