Rubber-based bungee cords do not hold up well against heat and tension
This morning I found the lighting fixture from my top shelf hanging down at a precarious angle. One of the two cords which had been holding it up had snapped, and the other one was fraying. I am very lucky that the lamp hadn't fallen completely, and that both the lamp and my plants are safe.
A more secure setup, using metal chain
It looks like the heat from the lamp caused the rubber inside the bungee cords to deteriorate. I needed a more secure way to hold the light in place. So I went to the nearby hardware store, and picked up 5 feet of metal chain. I wrapped this tightly across the shelf, and used my craft wire to join the ends together. One tangential benefit of using a metal chain instead of elastic cords, is that it added another inch to the lamp's height.