I did it on my '66 back in the day. Removed the back seat, covered the seat back area with marine plywood and built sort of a shelf. Then carpeted the whole area and placed the battery in a marine battery box where the right rear seat was. Used welding cable for power and ran it under the body up to the front to the solenoid. Worked great and gave me an excuse to not let anyone ride in the back.
1989 Blue LX Hatchback
2001 Red SVT Cobra Convertible (Phoenix)
2002 Black/Red GT (Ginger)
2011 Red Candy Metallic V6 (Manfred)
2016 Magnetic Metallic GT (Susie II)