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.
Problem with that is I have an OEM Mopar Cargo Mat under the Kicker Sub box. The OEM mounting is above the edge of the cargo mat so it doesn't interfere. Problem is you need a wrench or screw driver to unbolt the bracket(s) fro the sub box. I need to take a closer look at the OEM system and see what I can do. The drivers side rear seat back folds down but you can't move the sub box. Maybe replace the hardware fasteners with thumb screws or bolts with knobs on them.