To transfer design, draw a 15x10-inch rectangle onto a large piece of cardboard. Divide rectangle into 24 squares by drawing horizontal and vertical lines 2-1/2 inches apart. Working several squares at a time, mark the large grid exactly where the outline of the nation intersects each line on the grid, opposite. Connect the marks on the large grid with a continuous line, following outline on the grid. Cut out the design.
To make the foil pan, cover the cardboard shape with foil. Tear off about four more sheets of foil that are 12 inches wide and 16 inches long. Fold each in half crosswise to 12x8 inches, then to 12x4 inches, and a third time to make 12x2-inch strips. Snip or tear 1/2-inch-long notches on one side of each strip at 1-inch intervals. Folding notched edges under cardboard base, fit strips along the cardboard with the foil extending upward 1-1/2 inches. Tape the strips of foil to the underside of the cardboard base and to each other.
Grease and flour bottom and sides of foil pan. Place in a 15x10x1-inch baking pan. Pour batter into foil pan and bake in a 350 degree F oven for about 30 minutes or until a toothpick inserted in center comes out clean. Cool on wire rack. Remove foil strips, leaving cake on cardboard base.
Frost sides and top of cake. With a toothpick, mark a box in upper left corner of cake. With red decorator icing and ribbon tip, make stripes across top and down sides of cake, avoiding marked corner of cake. Place rows of blueberries in the corner. Makes 12 servings.