The diagonal of the top or bottom faces of the box is the hypotenuse of a triangle with legs 12 cm and 5 cm. The Pythagorean theorem tells you that diagonal length is ...
d = √(12² +5²) = √(144 +25) = √169 = 13 . . . cm
The width of the divider is 13 cm.
The height of the divider is shown as 12 cm.
The dimensions of the dividing rectangle are 13 cm by 12 cm.
