ports

I did a search but didnt come up with the answer to my question.

Some subs give you dimensions for a round port.  Can I do a slot port or square port instead?
If so how would I go about the math to make sure I get the measurements right so it doesnt mess up the sound?
Im not giving certain brands or sizes because its kind of just a general question about math.
OR
Is there a place where once I decide what sub I am using and look up a slot port size per that sub and enclosure size?
Im just asking because some compaines give you a square inch area X a length and some just give you a diameter X length.
Ive built about a dozen or so sealed boxes but havent done any ported because I didnt want to get it wrong, well I am ready to learn about ports and the correct way to incorperate them into the enclosures so any help would be great
Any links would also be great
Thanks
Bernie

I was about to read thru post to see if i could get the same answer your looking for .  For example If i have a port and recomended specs are  sw 1.373'' x sh 12.5'' x sl 23.75'' , am i able to make the port sw 2 x sh 6'' x sl (X) . Im not good at figureing these things out. but just want to know if they can be altered in such a way and still work as well as the manufacturer suggested size.

Thanks