inner_box = [15, 3, 20];
wall_strength = 2;




difference() {
  difference() {
    cube(size=[inner_box[0]+wall_strength*2, inner_box[1]+wall_strength*2, inner_box[2]+wall_strength], center=true);
    translate([0,0,wall_strength]) {
      cube(size=[inner_box[0], inner_box[1], inner_box[2]+wall_strength], center=true);
    };
  };

  translate([0, wall_strength / 2, (inner_box[2]/2 - wall_strength/2)]) {
    cube(size=[inner_box[0] + wall_strength * 2, inner_box[1] + wall_strength, 4], center=true);
  };


  translate([0, -(wall_strength/2 + inner_box[1]/2), (((inner_box[2]- (8+5.5))/2) -inner_box[2]/2 + wall_strength/2 + 5.5) ]) {
    cube(size=[inner_box[0]-3.5, wall_strength, inner_box[2]- (8+5.5)], center=true);
  };
};

translate([inner_box[0]/2+wall_strength/2,inner_box[1]/2+wall_strength,(inner_box[2]/2+wall_strength/2)-4]) {
  rotate([180,0,90]) {
    translate([-(wall_strength+inner_box[1]), -wall_strength/2, 0]){
      polyhedron(
        points=[ [0,0,0],
                 [inner_box[1]+wall_strength,0,0],
                 [inner_box[1]+wall_strength,wall_strength,0],
                 [0,wall_strength,0],
                 [0,0,-4],
                 [0,wall_strength,-4],
        ],
        faces=[ [0,1,2,3],
                [5,4,0,3],
                [4,1,0],
                [3,2,5],
                [2,1,4,5],
              ]
      );
    }
  }
}

translate([-(inner_box[0]/2+wall_strength/2),inner_box[1]/2+wall_strength,(inner_box[2]/2+wall_strength/2)-4]) {
  rotate([180,0,90]) {
    translate([-(wall_strength+inner_box[1]), -wall_strength/2, 0]){
      polyhedron(
        points=[ [0,0,0],
                 [inner_box[1]+wall_strength,0,0],
                 [inner_box[1]+wall_strength,wall_strength,0],
                 [0,wall_strength,0],
                 [0,0,-4],
                 [0,wall_strength,-4],
        ],
        faces=[ [0,1,2,3],
                [5,4,0,3],
                [4,1,0],
                [3,2,5],
                [2,1,4,5],
              ]
      );
    }
  }
}