Bake the cake according to the directions, in a 9 X 13 inch pan. (Use the high altitude directions if you need them.)
Cool. (Cake may be made ahead and frozen.)
Place on a serving plate or board.
Whip the cream with the confectioners sugar.
Cut the strawberries in half lengthwise.
Spread the cream evenly around the sides of the cake, saving a little for the top.
To decorate: Using a pastry bag, pipe (or dollop and spread with a spatula) rows of cream alternating with rows of strawberries for the flag "stripes".
Make a background of the blueberries in the upper left-hand corner, then pipe or dollop stars on top to create the "star" section of the cake.
If making cake ahead, arrange fruit several hours before serving.
Add the cream up to an hour before serving.
VARIATION: Use chocolate cake mix instead of yellow.