On your first question.
I would get some flat stock and cut a curved piece to drop in the channel and rivet it down. Then cut pieces to rivet to the vertical legs of the channel.
The fiber tubing ran from the tail lights to the front of the trailer at a location you could see in your rear view mirror. They carried a signal forward when the brake lights were on. A nice idea to tell you if you were missing a tail light but not a real necessity.
Short of buying a new rear quarter panel anything you do will look like a patch.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles