you always want the inside of the box to be sealed were the edges of the wood meet so that the air doesnt leak out causing the box to rattles or sound like garbage, poly fill is usually used to make the speaker think it is in a bigger box than what it is. as far as what john said about using a foam product, well i think in his case its diffrent cause he competes so his music has to sound different but as far as the average jow who just wants a loud, brain rattling stereo i think what you have now is fine, also due to the fact that you say your whole car is dynamated so, i dont see why you would have to alter your box.