I used 7/8" plywood, 8"x40", for the tunnel’s floorboard. Paint it to make it smooth. Cut a 3"x7" rectangle out of the center for the test section: first drill pilot holes, then cut it out with a jigsaw.
Make a base the same length as the floorboard but several inches wider, to give extra room for the control panel. I used a 1" board for the base. Line up the boards and trace the 3"x7" hole onto the base — this will help you align the test section later.