here is some info from rockport speakers that sums up a central challenge to building speaker enclosures...
"Our composite cabinet walls are constructed from two, high tensile strength, glass fiber reinforced, loaded resin shells that are separated by a proprietary high density, high hysteresis loss, epoxy core material that bonds the inner and outer shells together. This geometry creates an immensely stiff, yet highly damped beam section for the cabinet walls, exploiting the shell’s tensile modulus, and the core’s compression and shear modulus. This kind of composite construction cannot be matched by any single or homogeneous material since the cabinet walls must exhibit high mass, high stiffness, and high damping, a combination of conflicting attributes unobtainable in any single material."