My stooges are pieces of 3/8" plywood about 18" x 30", they work fine on grass without slipping and without extra weight or stakes.
BUT that's with a 52 ounce plane with an LA-46 running correctly. If I adjust that same engine lean on that plane, it starts dragging the stooge.
If you're on pavement then use a weight, and put something on the bottom to enhance friction.