When doing stereo install's I always use something like this, ($1 each from bunnings) to mount the sub in the boot..

Digi Online Store: Antenna Mounting Bracket - L-shaped wall mount. 12 mm hole. Compatibility: 76000843, 76000841

Drill into side of box and drill into boot floor, and keeps your sub easily removable if you keep a screwdriver in your glove box.. I went around barbs with 2x12" subs screwed down by 2 of these brackets and it worked a treat

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