The wood amplifier board I made up and covered. It is measured and cut to the exact size of the carpet edges. I mounted a piece of 34mm pine underneath and drilled out some under sized holes to the size of the factory screw points and push fitted it in. It's very snug when you see it I can show you. Once pushed in it requires a couple of firm tugs to remove and so won't go anywhere. The floor that goes over this is again cut to size and once carpeted requires considerable pressure to squeeze in to the edges of the boot so will again not go anywhere as acts as a secondary fix. The piece of floor that covers the sub is again an exact fit to the well.
E90 330i SE Auto Graphite Grey Metalic