Wanted to get some opinions on using polyfill inside a sealed sub box. What are the advantages/ disadvantages?? When airspace is a concern, would not stuffing the box full of polyfill reduce that airspace??

I am in the final stages of building my box and would like to get some input before I seal this puppy up.

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2001 X5 3.0i (PP/WP/AP) Traded at 213K miles
RARE White on Black - Why does everyone get Tan??
First E53 with 215s on XOutpost!!