Just a note to say that aluminum will not magnetically shield your speakers. You will need steel plates if you intend to shield the box itself rather than the drivers. Adding a modest bucking mangnet to the driver may be an easier solution and will only change the Q of the driver by around 10% or so. I doubt whether this will be all that significant given that you are working from published specs rather than measurements of your own drivers. The other problem with relying on steel plates, is that you might accidentally magnetise them during the build process and cause the very problems you are trying to solve!