Drilled the 1/4 inch hole for the Main battery (minus) cable connection to the airframe. Took the primer off on both sides of the mount point and spread a light penetrating oil over the exposed aluminum as corrosion protection. Bolted the cable to the aircraft frame and the battery (minus) post using locknuts. For the Engine Battery (minus) cable, I fabricated a mounting bracket for the airframe which required drilling out an existing rivet to the baggage rib. The mating surfaces of the bracket and the cable were cleaned of primer and lightly oiled. For this connection it was prudent to add an Adel clamp to provide strain relief and to protect the cable from abrasion. For both battery (minus) cable connections, I performed a continuity test between the battery (minus) post and various locations on the airframe -- all okay. ...After some thought, I decided to add an Adel clamp on the Main battery cable too.