4 x cones or markers, an old jumper/garment placed on the ground, as long as it is visible to the camera.
Cone A placed at entrance.
Cone B placed 8m opposite cone A.
Cone C placed at exit, 8m opposite cone B - 16m from cone A
Cone B.1 placed 6m parallel to cone B.
Cone B. 2 placed 6m parallel to cone B - 12m opposite to cone B.1
A chair or anything you can sit down comfortably on.