Woofer Box Calculator will help you design enclosure (box) for your car audio speaker (sub woofer). It was developed by 2012 and 2013 european champion in dB Drag Racing and was designed to be easy and transparent. It is better than other software because it also considers approximate cabin gain, power compression and port compression. Because of that, it can also predict final SPL in your car.
Furthermore, it knows how to handle total (gross) box volume instead of net box volume. It can also calculate vented (ported) enclosures in the opposite way (e.g. it can calculate vent size or total box size for desired vent).
Units an constraints can be changed under settings. User interface is from the PRO version, but some features are locked.