Doesn't matter what shape the cabinet is, you want your port length 36 inches long maximum. In your design of 223 liters net volume, a 6 inch port 36 inches long will tune it to 15.5hz and a 8 inch port 36 inches long will tune it to 20hz. To tune the sub to 15 hz with a 8 inch port, it would have to be 70 inches long, which is unacceptable.
Here is an online calculator from Precision Ports, it will make it easier for you to find the proper port diameter and length for a given volume.
http://www.psp-inc.com/