That opening is 45.96 square inches, actually pretty substantial. That said, I ran some numbers and that port length is tuned to right around 30Hz.
You should consider dropping the size down to 4.5 Cubic feet as according to the manufacturer, 2.25 per driver is optimum.
A 4.5 cubic foot box with a 2”x11”x10.75” (22 square inch area mouth no matter how you square or rectangle it) and 10.75” length will get you to 30Hz.
With a 45.96 square inch (3.17” x 14.50”) mouth, the length would need to be roughly 25.5” long.