Tonight I decided where to install the standby battery and made the parts to mount it above the center channel about 3 inches aft of the firewall on the cover panel between the sets of rudder pedals. I made backing plates to reinforce the panel and to hold the nut plates.
It was a pain to work on this panel because the canopy release cable passes through behind the battery, and I can’t remove the panel without undoing work on the canopy release mechanism. So I drilled, deburred and riveted everything with the panel nearly in place, which was awkward.