Draw a 12-by-4 inch rectangle on the poster board with a pencil, using a ruler as a guide. Add a 2-inch high triangle to the top of the rectangle to create a fence slat template.
Draw a 15-by-3 inch rectangle on the poster board to create a fence post template. Cut out both templates.
Break down the cardboard box into sheets of cardboard with the scissors.
Trace the fence slat template on the cardboard 10 times and cut out the cardboard slats with scissors. Trace the fence post template on the cardboard three times and cut out the cardboard posts.
Cut four 3-by-20 inch strips of cardboard.
Roll the air dry clay into three balls the size of a tennis ball and flatten them slightly. Insert the fence posts in the clay and let the clay harden for two hours. Position the fence posts 18 inches apart.
Attach the long 20-inch strips to the fence posts with the hot glue, positioning the strips horizontally three inches from the tops and bottoms of the posts. Attach the fence slats to the strips vertically with the hot glue gun.