Fortunately, there's a 2" x 4" crossmember under the trunk floor, right under where we put the battery. We install nut-inserts into the rail, even after the fiberglass is in place. It ends up looking like this.
Locating the rail with the rear suspension in place is going to be the minor challenge. It's under the back edge of the flat section of the upper trunk, just in front of the slanted part. You can substitute large sheet metal screws (at least 1/4") for the inserts.