Was in Kmart the other day looking for silicone baking sheets to make gaskets from and ended up buying a silicone muffin tray for $5. Not only was I able to make some nice gaskets but one of the muffin things also fits perfectly over the neck of a keg to make a handy seal. I just cut the entire muffin cup section out, trimmed it and jammed it on. A clamp also fits over it to get a better seal. Food safe and cheap.
A few images