the port can be made smaller, but the speed of air goes up. if using winisd, the rear port air velocity tab shows how fast air is moving. too high and chuffing results.
that is why i suggested the port diameter and length that i did.
there are other ported options however, a slot port for example or even a square port. none of these are difficult to calculate.
i was just suggesting the easiest possible solution, a single round port with no bends.
the cross sectional area and length are all that matter.
a 6" port diameter is pi*r^2 = 3.14*3^2 = 3.14*9 = roughly 27 square inches.
a 5.25" x 5.25" square port is also about 27 square inches. so you could make a square port that runs along the bottom and then up the back of the enclosure.
or a 2.62" x 10.5" port of the same length.