I installed EFII so wanted the redundancy of 2nd battery. Peace of mind.
Mounted second battery inside over the pilot rudder pedals on fire wall.
Just below structural channel with enough tilt clearance to take battery out and still have full rudder pedal travel.
Since I mounted my engine ECU's on inside firewall, I added access panels on both sides of the boot cowl. It then allows easy access to ECU and battery. And if u really wanted to move the rudder pedals, you could do it without standing on ur head.