The AirlockRiser from Marchisio is the #1 way way to prevent leaking and excessive headspace in your variable capacity tank. With the riser, you can completely fill your tank up to the riser, thus nearly elimnating the headspace in the tank. Less headspace equals less oxygen, which ultimately is better for your wine!
To complete the setup, you'll also need a #9.5 stopper (Item #4628), and a simple airlock (i.e. 3-piece airlock Item #4600).