To design and plan what kind of box I needed I used Google Sketchup to design it after working out what volume and port size/length I needed in WinISD. This gives the woofer a usable extension down to around 32Hz while keeping the box a nice 35cm/13.7" cube enclosure.

As you can see the port has a corner in it, this isnt idea but well worth it to get the lowest possible response

What your looking for in WinISD is a flat response with the lowest response you can get. Its a pretty advanced program so you are best off watching a few videos online.

Remember the volume of the port is excluded from the volume of the box (Add port volume and box volume together then calculate dimensions)

Resources

WinISD - To use this you have to know all of the thiele small parameters (T/S) of your speaker

For some reading on speaker boxes http://www.bcae1.com/spboxnew2.htmhttp://www.the12volt.com/caraudio/boxcalcs.asp#por...

As pointed out in the comments wall thickness of the port as well as a rough estimation of the volume the driver will take up need to be taken into account and a not part of the "Box Volume" WinISD will give you