For this to work in a 12 inch sonotube you would either fold the port or let it stick out above the sonotube like a stove pipe. In 2.5.cu ft tuned to 18 hz, with 500 watts input power and a HPF at 16 hz, port air speed would be 30 m/s at 17.5 hz with a 4 inch flared port 38.75 inches long.
Here's 3 different models. All with 500 watts input, HPF to control cone excursion, port lengths are 32 inches long and up. 2.5 cu.ft.tuned to 18 hz would give the most low end output at 20 hz and below for a HT application.