After a lot of research, I finally started building my 380L Tempest sub box. I had intended to use a 6 inch port, but with my 24 * 24 dimensions, it might be a little tight to fit in beside the big 15 incher. Should I go with double 4 inch ports instead? Also, is the placement of the 3 inch holes in the crossbrace an exact science? Can I put them wherever? Thanks.