Box design?

[Warpshift 10+ year member]

So I went over to Audio 2000 today and was given a quote of $580 after tax for a DD 12in and box. However my concern and I brought it up multiple times was that the box would take up the entire back portion of the trunk and stick out roughly 13in. I asked if they could make a smaller skinnier box that is ported on one end and is down-firing. I was told that they would need that much room to make it sound good. 

I just can't wrap my head around the fact that there seems to be no way to build a down firing ported box that isn't huge and needs to take up a ton of room in trunk. 

I found this post on another car audio forum, and I showed it to them, but was told it wouldn't work. So my question is why?

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[THATpurpleKUSH 5,000+ posts]

Step down to a 10 if you want to save space. And you never down fire in a trunk or any vehicle for that matter.