I used...
one of the white ones that you can pick up anywhere. It has dimensions a little larger than the sink opening, so I could screw on 4 small rubber "bumpers" on one side. Each of the bumpers fits just inside one of the 4 corners of the sink, so the cutting board will stay centered over the sink opening and won't slide around. This way you can use the cutting board while its on the sink, or if used on the countertop, the bumpers grab and prevent movement. Took the picture in the middle of remodel. Good luck