I have an atomic ele 15 and i want to build the box to specs but cant get the size right when i take off sub and port displacement. I got the specs from atomic and I want to build a 9 cubic ft box since i have the room. They said the port size needs to be 190si x 8.5 long @55 hz. My dimensions at 19.5x49.5x19.5 which is 9 cubic feet without displacements. Can any help me out because i need 9 cubic ft after all displacements