Yes! - If you'd like to have the option of using your Flip Bottle as a leak-proof, you can purchase a Series 2 Loop/Lid and Collar here.
All you'll need to do is unscrew your Flip Lid accessory, and screw the new Collar and Lid onto your bottle body. The Series 2 Collars and Lids are compatible with all sizes of Series 2 and Series 2 Flip Bottles.
Please note, you will need both the lid and collar in order to make the switch to a leak-proof bottle.