Adding polyfill or similar material would be a waste of time IMO. It will not add volume in any way shape or form. It may affect the response curve but that's about it.
Adding polyfill slows down the movement of air within the enclosure caused by the speaker cone. By slowing it down, it keeps the air cooler, thus, tricks the woofer that the enclosure is about 10% larger than it physically is. Thus affecting frequency response/curve.
The hack where someone cut in half the enclosure, added a wooden spacer, and fiberglassed it back together, along with a different woofer, that was definitely one way to improve things. But the time and expense of something like that or the lack of tools/materials/garage space... Might as well go get a custom setup from a local shop, try a DIY, or buy the JL Audio 10" perfectfit sub.
That said, there are a number of cost effective pre-made amplified subs you can look into acquiring. Often under $200. Or for just a smidgen more you can probably score a real sub, amp and LOC too.