Squeeze another .5 cubic foot net to equal 12 cubic foot net and do 8” aero ports (3) from big ass ports. Don’t worry about going against recommended specs, it’s not out of the norm to do this and companies use recommended specs as a guideline and they know and we know things vary on each application. The max length per port they sell is 22”. So on 3 of them ports, that would be 35hz tuning on 12 cubic foot net and about 12.285 square inches of port per cubic foot. Which is nice on an aero port build. Now let me figure the displacements of ports and subwoofer and I’m gonna add 0.7 cubic foot displacement for bracing. Subwoofers are about .23 cubic foot per subwoofer. And each port has a displacement of about 0.705 cubic foot. So to yield 12 cubic foot net volume, you have to add these figures to that net volume so you know the gross volume of the actual enclosure you are trying to get as close as possible to when typing in measurements. 15.275 cubic foot gross volume is what’s needed.