Changing the cab volume by a small amount won't make much difference. I suggest making the vent "adjustable" in some fashion, for instance temporarily installing the vent wall with some screws. Then you can measure the port tuning frequency and adjust as needed. When you're happy, finish up with glue.
You can measure the port tuning with a few grains of (uncooked) rice on the cone, and a signal generator such as is found in the nice free program Visual Analyzer. The rice will dance on the cone, and will stop dancing when the generator is set to the tuning frequency of the port.