Below is a link that took about 25 seconds to find on port design and flare design.
What l like a about this link is someone has actually done some experiments (the work) recorded the results and concluded with a port-flare calculator. The article is well written in and easy to follow.
http://www.subwoofer-builder.com/flare-testing.htm
http://www.subwoofer-builder.com/flare-it.htm