How do you convert circular port info into slot port or square port info?

I was just wondering how to do it. I have figured out that my sub needs a 4 inch diameter, 10 inch piece of PVC as a port. Well, I dont really want that kind of port so I was wondering, what do I need to do to figure out how big a square or slot port should be. I am not sure how to ask the question. Given the info on the port size I mentioned, how to I %26quot;change the port%26quot;. What would the deminsions of the port be if made into a square port??? slot port??/ email me if you need more info, since i am not sure what to put in here.How do you convert circular port info into slot port or square port info?
%26quot;If you want to use square ports, use a port who's height multiplied by its width equals the 'port area' on the calculator. The port length will be the same as the round port.%26quot;

http://www.bcae1.com/spboxnew2.htmHow do you convert circular port info into slot port or square port info?
The shape shouldn't play too much role in the sound,.. as long as you keep to the cu/in that the round port would provide you should be okay.

Also, I say the shape shouldn't play too much role, but it may. It'll probably be so minor you'd never know the difference though.