The holding bottle is the inner bottle that will be used to absorb the heat generated by the reflector. The outer bottle actually just acts as an insulator. The bottle itself offers little insulation. It is the air between the two bottles that insulates. In a warm climate, the outer bottle may not be necessary.
I used a 500 ml (16 oz.) large-mouth Nalgene bottle as the Holding Bottle. I coated it with several coats of black spray paint. This will aid in the absorption of the heat by the water in the bottle. Be sure to paint the bottom also. You could also scale this down with a smaller sized-bottle, but 500 ml of hot water is about perfect for two hikers.