Tonight I continued engineering a mounting solution for the various electrical boxes that will go in the "hell hole." I started by cutting the two base rails to fit between the existing floor angles. These will be attached to the floor with rivnuts and screws, and supported with bridging shims underneath, so they don't try to bend under the load they'll be carrying. Another angle will be riveted or bolted to this one (as shown in the photos) to form horizontal attach surfaces for the backup EFIS battery. Other boxes will be on small attach plates.
But that's only half of it. There will be a removable second layer, over top of the first one. I'm going to build a box-like structure with two levels. If I do it carefully, there will still be plenty of room for the alternate air cable and a few other electrical items to be mounted on the side walls.
I can tell there are going to be a LOT of wires in this compartment.
The backup EFIS battery, sitting on my proof-of-concept rail mount.
