That bracket should be good. Anything that will allow to go through the box to the chassis.
You going for active sub? that can be earthed anywhere, to bare metal and secured.
I had a external sub in my B6 that was earthed to the rear seat backrest fixing bolt. Had a amp attached to the box, so it's not flying around the boot and all neatly wired together.
Make sure you run the power wire on one side of the car and signal on the other. Not to get signal disturbance from power wire.