Dual-chambered with AP mat?

[HondAudio 10+ year member]

I have a Scion xB and I'm researching what to do in the back for a subwoofer. I currently have a JL 12w4 (yes, I know, OLD SCHOOL) in a typical wedge-shaped sealed box from my old car. It's 1.5 cubic feet net, and I was giving it 200 clean watts from Precision Power.

That said, I want to use the storage cubby on the right side of the trunk area, but it's too small to give me enough airspace even if I fiberglass. However, there's an area under the carpet that has a plastic filler piece to create a flat floor over the JDM spare tire well stamping, and I was thinking of using that as a secondary chamber filled with polyfill, with an AP port between the two areas.

Will this work? Will I get the deep, smooth extension of my current box, or will the two chambers together still be undersized and create a construction nightmare? //content.invisioncic.com/y282845/emoticons/uhoh.gif.c07307dd22ee7e63e22fc8e9c614d1fd.gif

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[Volenti 10+ year member]

The suspension on that old girl is probably getting a little tired by now ( I have several JL's of similar vintage) so an ap mat may not provide sufficient control over the cone(compared to sealed). Is the secondary chamber going to sealed as well as a normal enclosure or will it be "leaky".

If possible I would try and make the secondary chamber as well built and sealed as the primary one and combine them directly, with both packed with fill I doubt the weird/extended shape will cause any issues and would certainlly be better than the smaller chamber alone.

[edit] and yes it'll be a construction nightmare.