Were does it say in the code that you must use a Fan box. The way I read it you need a fan rated box only when it is the sole support of the fixture.314.27 D
So if you used a regular steel Oct box For example and attached the fan bracket to the box with the 8-32s and also ran some long screws up threw the box into some good wood you would meet code. There are some pretty cheap fan boxes out there now. that basically do just what I am describing and skip the 8-32s also. Safty is #1 you got to know that fan is not comeing down so be safe and use screws other than drywall screws.
