Hydro Flask Small Bottle Boot Hydration Belts & Water Bottles Cobalt : The Hydro Flask Small Bottom Boot lets you personalize your favorite hydro flask bottle while offering a durable silicone base for added protection. Mix up the colors to match or just to compliment your current bottles hue. BPA free. Offers extra protection for your Hydro Flask. Fits onto 12-24 oz wide and standard mouth bottles.