By doing a quick calculation if you make the 18h x 38w x 15d and make your port a total of 30" it brings you to 31.77hz before displacement. After sub displacement it will bring that up a little bit. Currently your port is 25" long to acheive 30 you naturaly would extend the 2nd port wall 5" longer. I use this calculator http://torres-box-tuning-calculator.software.informer.com/ Works great.