We assume the base is 12 in × 20 in, so its diagonal has length
... √(12² +20²) = √(144 +400) = √544 = 4√34 . . . inches
Then the tangent of the angle of interest is the ratio of the box height to the base diagonal:
... tan(α) = (7 in)/(4√34 in) ≈ 0.300123
The angle will be the arctangent of this value,
... α = arctan(7/√544) ≈ 0.291569 radians