Clip a locking carabiner through both the groin loop and the waist loops of your harness (the same loops you tie into). Take a loop (bight) of rope from the end which the climber is not tied into and ...

The bottle is mostly made from stainless steel, although the very top bit, which creates the twist beneath the cap, is made from plastic. Size-wise, the bottle is fairly chunky for 500ml.