Like Velcro? Lots of little hooks that hold onto the carpet?

Yeah, that's how my sub is held to the floor of the trunk right now and it works like a charm.

Note that it only works because I have a hatchback-shaped box (sort of a pyramid shape) with a flat bottom. There are some subwoofers shaped like a cylinder, and those require mounting straps (which come with the subwoofer).

Regarding the idea of building your own: Remember that you can purchase prebuilt boxes like the Q-logic ones and simply drop your own speaker into them. You can get sealed, bandpass, whatever. That's what I did and it was quite easy.

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Tony Fabris