a small scrap of printer paper
one ball bearing (N= 0.075" / HO= 0.138" / O= 0.25")
glue - I used Gorilla Glue for wood
Twist the paper into a slim cone and glue it. Use a wire to spread glue on the inside for additional stiffness and strength. Trim cone to height when dry, so it looks convincing scale wise. The cones are about 3 feet tall, so in N the cone would be 1/4" tall, in HO a shade under a half inch, and in O make the cone 3/4" tall. Glue the ball bearing onto the cone. Place on your layout.