Polyfill's main purpose is not as a quick-fix to an enclosure that is too small. If that is the intended reason for its use then serious revisions need to be made to your design. Its used to stop standing waves inside an enclosure. The possitive "side effect" is that it fools the driver (and sometimes the listener) into seeing more space as the soundwaves get translated into heat instead of reverb. There is an audible difference between polyfill used to decieve and polyfill used to enhance. I believe the general rule of thumb is 1lb. per sq.ft.