The light bar was for load, and an excuse to do it, and ...why not? That's three reasons.
I forgot about the 3rd brake light. Eight 18650 cells is only about 24" isn't it? Those could easily get lost on the top, but with 180 cells you'd want them lower down. It's a long way from the 3rd brake light to the battery. Your battery pack could go anywhere in between.
I was picturing the LED bar being no higher than the roof rack. If you only radius the sides and rear you might get down to 8-10sq ft. Strips of aluminum roof flashing would be adequate if they were rolled.