When I first wrote about my rear seat belt modifications, I initially planned to mount the inertia reel below the floor, and then cut a slot in the floor to route the belt up to the shoulder anchor point. However, the more I thought about it, the more I leaned towards a simpler install, and so I selected a location on the beltframe behind the seat, just above the luggage floor. It gets neatly tucked into the corner to minimize the amount it sticks out. To mount the reel here, I designed a corner bracket, which is made from 0.08″ aluminum (same as the hat section for the shoulder anchor). My CAD model grew to include more of the belt frame, so that I could both design the parts and then verify their strength.
After completing fabricating the parts last week, I installed the new seat belt shoulder anchor as well as the inertia reel bracket, completing the starboard side. I need to install the same parts to the port side.
One other change I incorporated was to move the outside lap belt anchor forward, to have it better line up with the forward location of the rear seat.