I have a 10" round PVC port with 2 15's. Cant remember the length. Cut your hole close to the OD of the pvc pipe and epoxy it to the box and finish. I added a coupling to the inside part of my port and uses a round over bit on it, actually a Corian sink round over/angle bit. Used it on the outside of the box too.
With my 10" port I dont have any port noise with my 15's.
good way to keep it from coming unglued is drill holes at the end of the pipe and holes in the wood so the glue grabs good. hot glue works well then silicone over it.
I plan on doing this for my box as well, 2 8" external ports for an 18" APXX for a total of 100.5 square inches of port area. There are some drain adapters that work really well for flaring, and for connecting on the box, though I don't have a lot of room so I'm just doing a similar direct glue method mentioned above.