hi man, basically you make the port before you make the box.
lets say you had a 30hz tuning freq - and you needed 3ft^3. build your port to that box size and tuning- then create the fiberglass around it. hopefully when its done, the net volume inside (after all displacements) will be more than your desired 3ft^3. fill it up with somthing (packing peanuts work very well) and then measure the volume of the peanuts add piece of 4x4's or 2x4's until the desired volume is reached.
then put the sub in and bump away. its always a good idea to tweak it after too- try taking the box volume down a little more - say 2.8ft^3 and see if it hits harder\sounds better, or try making it 3.3ft^3 and see if you like it more.
but thats generally how i did it.
=)