Please note that the dimensions for the 12"x12" pieces should be modified as follows:

Two pieces should be cut for the 12" sides and two pieces should be cut for the 12" ends at the back of the box.  I chose to cut the end pieces to reflect the thickness of the plywood since the above configuration will leave you with either a gap or an overhang.  I made the end pieces to fit inside the sides and the upper and lower pieces.  If you want the ends to basically 'cap' the ends, then you should add 2Xs the plywood thickness to these pieces.