I put a doubler on the floor behind the baggage compartment right of the keel, and two "Z" structures made of angle riveted back to back for the mounting structure. The outer flanges of the "Z"s are riveted to the doubler and fuselage skin (countersunk, dimple, dimple respectively) the inner flanges form a standoff mount with platenuts for AN3 box mounting bolts. The ELT is mounted on the floor on the left side of the keel in similar fashion. With a RV-6A using an O-360-A1A and a Hartzell C/S prop the CG is right on the specified location with no balast.