We haven't poured concrete below grade in a paved parking lot before this project.
We cut 10" pieces of plywood and screwed them into the asphalt using 2-1/2" deck screws.
We screwed a 2x4 to the plywood along the parameter of the pour.
We pulled the plastic sheeting over the 2x4 to prevent the concrete from escaping.
We built a form on top of that 2x4 up to the finish we needed.
We put plywood on the face of the form and extended down to the existing grade.