This came up a while back and someone mentioned cutting up an old wet suite. I didn't have an old wet suite so...
I made one out of goop. I've used goop to make gaskets for swing keel bolts in the past as well. It works amazingly well. See photos. The first is the masking tape damn that I made around the ballast thank valve cover. Then I filled it in with goop and let it harden. I did not get a good pic of the finished product but in the third photo, you can see the gasket I made for a swing keel bolt on a previous boat. If you clean the metal on the inside surface of the ballast tank, the goop will stick very well to it and make a nice rubber gasket. You can get goop at Lowes.