Pit Detail. This is one way to construct the pit. The 2 x 6 forms are of Redwood or Pressure Treated and are not removed. The forms hold the concrete platforms in place.
Another way would be to construct the forms with a crossover (formed for concrete) at the front and rear and one strand of #4 rebar to hold the platforms together. When poured and cured, the forms may be removed. The concrete or plywood under the clay (concrete is better)also helps keep the clay moist. The plywood will rot out and the dirt will draw the water from the clay. {3' wide conveyer belt also may take the place of plywood or concrete.}