Got a old school Rockford Fosgate HE RFP3812

[bassiksauto]

First off I know it is an 8 ohm sub but this young fellow came to me I found him a sealed box but I want to make it ported the box is 2.5 cubic feet and I'm just wondering what diameter and length of Port would sound best. yes I'm using an old school Round port as I have a lot of ABS around here could somebody give me a pointer please and thank you I would like to get this thing done quick so I can give it to the neighbor kid

[bassiksauto]

Fellas any type of help would be appreciated as this young fellow wants a lot of bass and he does not have much money so I picked this sub and box up for free and I want to see if I can get a little bit more output from it. I only install car audio I do not build boxes as I don't understand the fundamentals and if I do build a box I basically need all the specs and then I can build it