Aaahhhh....sub boxes!
I had the same problem as you did RedSTA...i made a box, for 2 shallow mount Kickers and in the end, it was OK, but there was not enough volume, they shared the same airspace, and it was just not built 'right'...(my first ever try at fiberglassing). Here is what it was:
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it was a clean setup for a first try, but you could 'hear' the subs choking out on faster beats.
My second attempt was what i have now, and much better. I used the entire back wall (4 ft wide), and went 13 inches high. Thanks to some good advice from members like psycareyo, i also seperated the box for 2 sealed enclosure, i for each sub.
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To give you numbers, my first box was 1.2 cuft for BOTH subs. the new one is 0.9cuft PER chamber. The sub requires 0.8cuft.
I was able to achieve this using fiberglass i must admitt. 3/4 mdf will cut into both airspace and your mounting depth. my fiberglass backwall was 1/8, which was 9 layers thick.
and a note on polyfill. What guys have said above is true. There is no 'AI' as far as i know in either the polyfill or the sub, so nothing is 'fooling' nothing. it's a myth that you can compensate for cuft of airspace with it...that being said, there is a value for adding upto 50% of polyfill of your airspace, because as Dingo said correctly, it eliminates standing airwaves, and also causes waves to resonate a bit more as there is substrate for them to reflect off. this helps is providing a bit of 'richness' (for a lack of better word) to the output, but it does not make your 1 cuft enclosure into a larger one....
My build log is here below...it's not hard, can be done in a weekend, and i can help if you have any questions!!