Apology for being late to this, but if DIY is acceptable to you, there are many resources if you wish to learn about them. One caution, though, is that the words/labels/semantics get in the way very often. If it is a complete, ready-to-use speaker, of course, please ignore. If you are more interested in experimenting with such geometries for low-frequencies, it is quite simple to get started (and read more as you go). When adding multiple drivers and crossovers and so on, it becomes much more work and is best to follow someone's well-established project. Often, people start with smaller, more economical "wide range" drivers to become familiar with the process--even prototyping in disposable materials.
