My understanding is that they use fiberglass (as many custom car subwoofer builders do) because it can be built to be extremely rigid, but allows for any shape. The JL box uses some unique curves that would near impossible with MDF (except for B&W!). I forget where, but I saw a thread on a tour of the JL plant and they show the how-to on the Gotham build. Very interesting, but not really DIY type stuff. But for $12K I guess you don't expect to have a souped-up DIY project.

Panny 3000 PJ, 118" Carada, Denon 3300, PS3, Axiom QS8, PSB 5T, B&W sub, levitating speaker wire