Make your Hydro Flask your own with a colorful, durable silicone Flex Boot. Customize your look and protect against bumps and dings. Medium boot fits 32 and 40 oz wide mouth bottles. Product Benefits: Removable, silicone boot adds extra protection to your Hydro Flask. Medium boot fits 32 and 40 oz wide mouth bottles. Medium boot is not compatible with 32 oz Lightweight Wide Mouth Trail Series bottles. BPA-Free.