In this video, I explain how to model a ported subwoofer enclosure using Jeff Bagby's Woofer Box and Circuit Designer, a free software and manufacturer's published T/S parameters. The whole process is much simpler than you might expect. My recommendation is to follow along with the video to learn how the software works and then play around with a bunch of of different woofers and box tunings to see how different parameters affect your response. The more you practice, the better a feel you'll get for the things you can do to alter the response.
The subwoofer I'm modeling is the Dayton Audio RSS265HF-4. I love this subwoofer! I've built a 3-way with it and a traditional subwoofer with the 12" version. It is very low distortion and clean sound and you definitely won't beat it for the price. Anything from the Dayton Audio Reference line is very good quality and definitely something to consider if you are building a DIY project.