This will do just as well.

Place the milk box on ground midway between the lanes on your finish line. Align the box along the track axis by sighting between the "V" shapes of the latticework on the box centerline. Aim up hill to a point midway between the start ramps.

Step over to the sides of the box, and sight through the centerline "V"s again, Direct your pole men with broom sticks to place them correctly. First one lane, then the other.

If this is incomprehensible, just give it to any Cub Scout that is good with a BB gun, and he will have the finish line marked out in no time.