I used the flapper to cover the bottom (outside) of it and used silicone to hold it in place. As backup, I used a Gatorade top and cut the rim in half so that it wouldn't stick so high up(from the size bottle shown below) to cover the hole on the inside ... again using silicone to hold it.
<img src=http://dining.cornell.edu/imagesX/C-store/gatorade.jpg>