Mounting subs free-air like that will work, but you need to take into account the following:

1) The speakers must be specifically designed for free-air use. You have to check with the manufacturer or the dealer to make sure. If you don't, you can blow the subs, or at least shorten their lifespan significantly.

2) The trunk (oh, sorry, "boot") becomes the enlosure. There must be an airtight seal between the cabin of the car and the trunk. As well as around the edge of the speakers. The front of the speaker cones are inside the car's cabin, the back of the speaker cones are inside the trunk.

3) The trunk itself must close and seal tightly.

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Tony Fabris