Simply take a two litre lemonade bottle (cola, Fanta and Dr. Pepper are also fine) and cut the bottom off.
Next take a rubber glove and cut off two of the fingers. Now cut the tips off of the fingers so you've got two short pieces of flimsy rubber tube.
Stretch one of the tipless fingers over the neck of the bottle. Stretch the other tipless finger over the one already on the bottle, so you've got two layers. It should now look like this.
That's it! The double-rubber provides a cracking seal in the expansion tank, and a tall two-litre bottle provides ample room for top-ups.