[Mike P.]

If the overall internal volume is 12.3 cu ft then the net volume would be about 10 cu.ft. Two 6 inch ports 32.5" long would tune it to 20.5 hz.

 

[Mike P.]

The port air speed is in relation to how much power is applied to the sub. Knowing how much power the sub will see will determine the proper port diameter and length. What will you be powering the sub with?

 

[Mike P.]

10.5 cu.ft. tuned to 17.5 hz would make 2 six inch ports 42.8 inches long each.