The reason ports weren't included in your kits is that they aren't cylindrical, and are instead made of wood. I include material for cylindrical ports except for speakers that don't need ducts and those that have rectangular ports, which are made entirely of wood stock. Just make'em out of the same wood material you've used to build the cabinets. Make sure the inside dimensions are 4.25" wide x 2.5" tall and 3.25" deep. You're right that the cabinet volume can change by a few cubic inches and there is no way you could tell, but the port size is more critical. It's smaller, so there's less tolerance.