Looks like there's some Bounday Element Modelling software available for us to play with. Those kind folks at the University of Rome have GPL'd their in house system solving code. It takes a set of patches (or simple shapes like planes and cylinders) and solves the wave equation for interior or exterior problems. I think it can be used with an open-source 3d designer program to actually build the models. But there's no customisation for any of the specifics of speaker cabinets that I can see (could be wrong). Of course, this could be added.
I'll update when I get a chance to really look at it in detail.
acousto.sourceforge.net/index.php
