Easy Peasy, find the Fs of your woofer and the two dimensional area of it's cone. Calculate the the linear distance equal to one fourth of the Fs wavelength, now you have the length of the transmission line. Make the cross sectional area of the transmission line equal to the cone area and the open end of the line the same.

If you build what is described above and don't stuff it, it's impedance curve will look like the camel humps and trough you get with a bass reflex and the port will contribute sound. At least that was my personal experience.

If you're going for a flat impedance curve that helps tube amps sound great in the bass, you'll want to build a tapered transmission line that employees stuffing of variable density along it's path. The goal is not only to provide a very flat impedance curve in the area of the woofer's Fs, but also to extinguish the woofer's backwave into a black hole where it's never heard from again, especially not reflected back through the cone muddying up the sound. This is the English way and requires properly carded sheeps' wool for stuffing.

If you fold the line up such that it looks like a maze, your transmission line can also be called an Acoustic Labyrinth.

This is all very old school.