Find a similar sized driver and use it. Fi Car Audio has their driver displacement. For a port, use basic geometry. Measure the internal length, width and base that is taken up by both the port area and the wood that makes the port. FMOD is designed to work with a specific amplifier input impedance. So if you is designed to work with a 10k ohm impedance and your amp has an input impedance of 20k ohms then the filter will be halved (or doubled). I can't remember if change is inverse or proportional.