Add to your Hydro Flask Wide Mouth Water Bottle with the Medium Hydro Flask Flex Boot. Constructed from durable silicone, this flex boot allows you to customize your Hydro Flask bottle while also protecting it against bumps and dings. The medium flex boot fits either 32 oz. or 40 oz. Hydro Flask Wide Mouth water bottles. Keep your Hydro Flask water bottle protected with a Medium Hydro Flask Flex Boot.