I have used several online calculators to try and figure out how to do this. I have one audiobahn 1208t and for a round port in a 2.5 cubic feet box you need a 4" diameter port 8.5" long. I asked the audiobahn guys what the port area would be if I wanted to use a slot port and they said 25.5" area 8.5" long. I can't figure out how they got 25.5" opening. And I would go with that but I want to know how they got that so I can go with the smaller 1.5 cubic feet design which uses a 3" by 7.75" long port but I don't know how to correctly calculate it for a slot port. I read the enclosure design thread but I can't get my port to come out the way audiobahn says it should be. And audiobahn went out of business so I can't ask them.