Draw a rectangle that is 18-by-10 inches on a sheet of single-ply cardboard. Cut out the rectangle. Paint one side of the rectangle using spray paint or cover it with a sheet of contact paper.
Turn the rectangle so the 10-inch width is horizontal. Measure up 6 inches from the bottom edge of the cardboard and make a small identifying mark with a pencil.
Stencil the grade level, teacher's name, school or any other identification with a black permanent marker using the space below the identifying mark; you will have 6-by-10 inches to write on. Alternatively, apply self-adhesive letter and number decals rather than stenciling.
Cut two 10-by-6-inch rectangles from a panel of an empty cardboard cereal box. Stack the two rectangles on top of one another. Wrap heavy-duty tape around the width of the stacked cardboard to hold it in place and make it sturdy. Cover the taped cardboard with spray paint or contact paper.
Place the 10-by-6-inch cardboard on your work surface. Bend the larger rectangle so the decorated side is facing out. Position the curved board with the 10-inch-long sides along the 10-inch-long sides of the flat board. Squeeze hot glue along the seams between the two pieces.
Cut out two 8-inch square pieces of single-ply cardboard. Place one of these on the work surface. Turn the mailbox over and position the back end on top of the 8-inch square board. Trace around the perimeter of the mailbox and cut out the semicircle shape.
Use the semicircle shape as a template with the remaining 8-inch square piece of cardboard. Trace the semicircle and cut out this shape.
Position one of the semicircles on the back of the mailbox. Apply hot glue all the way around the seam between the back panel and the mailbox.
Place the other semicircle with the flat edge on the bottom edge of the front end of the mailbox. Apply a strip of heavy-duty tape along the seam between the semicircle and mailbox so the mailbox lid can be opened easily.
Cut a 1-by-3-inch strip of cardboard. Bend the strip into three distinct and equal sections. Position the shape as an upside down U and hot-gluing the back of one side to the lid.
Close the lid by pushing the lid gently inside the mailbox so it holds its place. Remove the lid by pulling the handle on the outside of the lid.