making a sealed box ported

[roycealbert1]

i have a custom made box in chevy crew cab that is for 2 12s sealed. the subs i have right now are sundown sd 12s. but i just got a fi sp4 18 that i have siting in a seat. and my buddie is going to buy one of my 12s so i was wondering what would be the best way to make the box ported for a single 12. the box is 2.25 cubic feet and its a common chamber. one method i was gonna do is make a passive radiator out of my old blown power bass 12. would that be good enough or should i make some type of aero port in the box.

[Problematic]

I would sell both sa12s with the box and build a new one for the 18.

Do that fi some justice