I don't think I saved the port measurements, but I basically just cut the top, bottom, and sides to the dimensions listed in the pdf and calculated what the dimensions of the port would need to be based on those sizes.
I don't have any real way of measuring it, but from listening to test tones I think the pair I built starts to drop off somewhere around 25hz, but I do seem to get some decent output at 20hz. I don't have any issues in the 30-40hz range, I'm getting great output in that range.
I've not had any issues with it sounding sloppy either, in fact it was a nice improvement from the Klipsch RW-10D sub it replaced in sound quality and it plays much lower.