Best thing to do is get on a build your cabinet, use a piece of plastic drain-pipe or similar for the port and cut a number of difference lengths and have a listen or measure the response and find which one you like.
Polyfill increases the apparent box volume, Adire have spent time optimising their design for the Tempest and have managed to reduce the cabinet size by using polyfil.
Here's a link you might find of interest:
http://www.win.net/audtatious/audio/fiber.html
Although the PE driver is a clone of the Tempest it works in a smaller box, if you decide to use the Adire plans you will probably need to adjust the port tuning. Don't get too hung up on the technical details at the moment get on and build the box, you will be very happy with a wide range of port tuning frequencies, but will need to find which one works best in your room.