if its 8.5 net ( after sub and port displacement) i would buy three 6 inch psp aero Ports and just throw them in your box. that will be right amount of port aera, tuning near 34 hertz and its easy.
It's the zcon 18. I called the company and it says it only has to have 6 but it would be better for 8 or bigger. But if I just made it the height of the box and like maybe 5 inches wide with the correct length for 34 hz, would that be good?