I used my Box Program and with 1 12" in a 2.5 Cu. Ft. box Net volume after subwoofer and port size taken off or 2.712 Gross Vol., 2 3" ports 9.3" long will tune you to 35hz., or 3" port 3.9" long for single port.

I think 3" ports will be fine for your subwoofer and box combo.

If you need any more combo`s or tuning tried just message me.