Casing designs differ from 1 another. That's why nowadays go for those case with 120mm fans for the front and the rear.
I'm using 1x 120mm fan for intake and 2x 80mm focus flows and 1x 92mm focus flow for case ventilation .. all controlled via a fan bus to minimise noise.
BTX form factor are available on the market but not for the average consumer. Its been implemented more towards business class PC.