When I secure a sub box, I use L brackets from the hardware store. Layout where you think 2 or 3 L brackets should go and then make certain the screws will not penetrate anything but sheet metal (i.e. brake line, spare tire, etc) Use the smallest length screw, pre-drill the hole.
After the sub box is secured, remove each bracket and paint them to match the carpet color. This will soft tone them, also paint the screw heads.
Re-install the brackets and they won't stand out.