When I was campaigning my salt flats race car, I wanted to know it's frontal area, so I used the shadow method. At the time, I didn't have a way to drive it (not street legal and/or engine not in it at the time), so I had it on my flatbed car trailer.
Then I took a sheet of particleboard and built a simple brace so I could stand it up at the back of the car trailer behind the car. The board and brace was held up and held together by c-clamps and "quick grip" clamps. Then I went to a place I could park it facing the setting sun and jacked up the hitch until flatbed of the trailer was aiming at the right angle up (the sun was still a few degrees up).
I used the symmetry of shadow on the board to help get the aiming right. Once it was right, I quickly traced the outline of the shadow on the board with a fine tip marker.
Then I drew a grid on the board, spacing the horizontal lines one inch apart. Then I just carefully measured the lengths of each of those lines, wrote them on one side of the shadow outline, and added up the numbers. The result is what I used as the frontal area of the car in square inches.
....To get square feet, just divide by 144.
