The first thing to do is remove the boot carpet and draw round it onto some (18mm) mdf and cut out to make a false floor.
Bolt that down to the boot floor, making sure there are no brake lines in the way underneath the car (I dont think there are) I used screws to do this, purely so I could undo them if I had a puncture, and cover the mdf with the carpet
Then use some corner braces, you can get these in Homebase/ B&Q for a couple of quid, then screw them to both the sub and the false floor, it is best probably to use at least 4 for this task, using woodscrews, but make sure the screws are shorter than the thickness of the false floor, or you'll puncture the spare.
If done properly, this will not only stop the sub moving around, but will also improve the bass response of the sub, as it wont move the box, just the air.
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