...I added a DB-9 under the switch for the serial feed to make updating easier....
I forgot to address this part of your question. If I were building again, I would cut out the back of the AirGizmo, make a small aluminum box to hold the two DB-9 connectors and pop-rivet the box to the back of the AirGizmo.
That way, when you are ready to do an update, just remove your Garmin X96 and plug your computer into the DB-9.
In my case, I riveted a section of the throttle bracket (since I didn't use it) to the sub-panel, over on the right side (No one has even kicked it, so it is well out of the way.), and cut/filed holes for the DB-9's, and then pop-riveted the DB-9 connectors in place.
Click on this picture. (They were labeled appropriately the week after the installation.)
(The cable to the right of the DB-9's is for the cabin heat.)
Of course, you could just cut, file, and rivet them to some out of the way place on the sub-panel. In this location, I can stand outside the plane, open the canopy, and reach in to plug in my computer. No need to climb in when doing the updates.
PS. I don't see a dimmer on your panel. I used it to control the radios and post lights I added for the ASI & ALT gauges as well as the light strip over the recessed switches. Just another consideration.
PPS. Think about adding at least one power port AKA cigarette lighter plug. I have two (from West Marine) and wired one to the always hot buss with thicker wire and a 7.5A fuse. That way I can charge the battery w/o opening the cowling. The 2nd plug was on the avionics master.