http://www.mcsquared.com/wavelength.htm

The link will give you the wavelength of a given frequency. Just use 1/2 or 1/4 for the length of your tube.
A transmission line is always tuned to 1/2 or 1/4 of the length of the wavelength of the resonant frequency of the speaker used.
If you want to use 1/2 the wavelength for the length put one tube inside the other so you have a folded transmission line. Of course you put a cap on the outer tube. The space between the inner and outer tube should be the same volume as the inner tube.
The wavelength of 40Hz is 28.25 Ft long. So 1/4 would be 7.0625 Ft.
You don't need anything else but some loose packing for the tube.
This will work with any woofer just adjust the tube length for the resonant frequency of the speaker.
You will need to build one of the 2 Eq networks. I built 2 of the active ones as I don't like crossovers.
Horns are not my forte. They are just to hard to build.