I plan on building a set of Dayton UM-18's whenever they finally come in. Box design calls for 2 - 4" ID ports by 19.84". A comment was made on PE that this 4" flared port will fit inside a pvc pipe - Part # 260-403. I haven't ordered them yet, but I am getting ready to and hopefully they will fit inside a pvc pipe which is much cheaper than buying that long of a port tube from PE. Another option is to make the flare on the end of a pvc pipe yourself. I saw a pretty good youtube video of this. Just heat the end of pvc until it is pliable and shape it over a bowl.
In my past subwoofer builds I have always mounted the flare on the inside of the cabinet, I like the look better. Cut your hole to match the flare, roundover on the outside, rabbet on the inside so the rim sits in the rabbet and PL glue it up really good.