In article ,
tommy wrote:

sounds interesting but is the design of the box hard? please give more
detail on how to build one


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tommy

There isn't a box. The back of the speaker vents into a large area such
as the trunk or the exterior; infinite baffle. The mat covers the vent
for acoustic dampening. The trick is that with no box, you have to make
a seal between the interior and exterior of the car. That means a large
air-tight custom panel that replaces the rear seats or a side panel.

It might not be very easy to install but it should sound very good.
Most car audio sub enclosures sound like crap because making them 4 to 6
cubic feet isn't practical.