Couldn't you use an old rubber ball from a thrift shop. They have different sizes, the heavy beach balls is what I'm thinking of. You could cut it in 2 & grease inside with Vaseline so it would release, to keep it round while pouring it should be in sand or dirt almost to top, I'd put a large rag under it to keep dirt or sand out of mold. Could use an old bowl in the 1/2 round shape be sure to grease very well, or even an old aluminum container, I had a lid for popcorn maker, soup warmer at college that would have been perfect! So look around your garage, basement, you probably already have something you could use. Jan