so 3k for a single 18
make some test boxes. one with sub and port facing the same direction and one with sub forward port side
this will allow you to test every sub port combo except sub forward port back (usually not good unless in a hatch, crx)
once you find which orientation the vehicle prefers start building boxes.
i like to start big and shrink the box until you stop gaining.
burp boxes are trial and error. so start trying