I found a rock to fit inside each crock. It must be free of moss or any organic material. I scrubbed them with hot water and then poured scalding hot water over them. Don't use any soap or bleach. After that they were boiled for 20 minutes and kept tightly covered until use. It will hold the plate down and keep air from getting to the sauerkraut while it ferments. Depending on what kind of rock it is, normally it is not to hard to trim a rock to fit by chipping the edges with a hammer. Be sure to protect your eyes from the chips which will fly off if you need to do this.