Your box, according to page 1, is to be 19" deep correct? Making the inside depth 17.5". Thus you were correct in your first port wall measurment of 15.5". I was mistaken earlier, It was late and I did it all in the dark, lol. And my original calculation for the second wall was correct of 5.75", lol. So in conclusion (lol), The first port wall length should be 15.5" and the second would be 5.75"