Not necessarily, RPG Modex Plate is 4" thick...I would still call that broadband since it operates over a wide frequency range, not narrow like a tuned trap.
Yes, but they are thick in price ...
I was looking at their specifications - they are really impressive. But they are very difficult to get in Europe, although they were designed at the Fraunhofer Institute in Germany. I think I will have to design a custom bass trap and tune it. Happily I have all the front wall space (13'x8') and no problems with depth.