Nope, haven't covered that.
The bigger panel (facing the rear seats) is braced to the big bracket that the 2 seat backs attach to in the middle of the car. I bought just a generic metal brace at Home Depot for 50 cents and bent it to the shape needed to go from the amp panel to that bracket. I wanted to brace things in a way that put as few holes in the car as possible, especially in the floor of the car, don't wanna do that at all. So using that bracket was perfect.
Then of course, the sides of the amp panels contour to the side panels in the car, so that eliminates side to side movement.
The sub box is attatched to that panel with a single long sheet rock screw, you can see it in the 4th pic down 2 of my posts ago. I also am still trying to figure out the best way to attatch the smaller panel to the sub box to make things even more secure. I wanna do it in a way that puts as few new holes in the box as possible. I also need to attatch it in a way that allows some twisting/swivel movement at the bottom of the small panel but no straight forward or straight back movement, so I can pull the top of the panel forward and reach down behind it and make the lower attatchment, then put the top back up against the bigger panel and screw it down lastly. You can see the 2 sheet rock screws I used to attatch the top of the smaller panel to the top of the larger panel in that pic you quoted.
So, things are secure enough so that it stays in place, the box won't fall over or anything, but I want to eliminate weight from
shifting as much as possible, because shifting weight essentially adds weight to the car when taking off or turning hard.