The issue (from what I've seen) isn't between the box and the carpet, but the carpet and the metal of the body. My box will stick to the carpet covering the body and spare well, but the whole carpet moves against the metal of the body. Velcro between the body and carpet would work, so would l-brackets, and of course a false floor.
I went with the false floor route since I'm re-doing my system with smaller subs and a cleaner look. I even made a little hatch that you can pull out to access the spare.
Here's some pics taken from my thread
here:
Here's the frame being fitted:
And then after being covered with 3/4" MDF and filling in seams/screw holes:
And finally after being carpeted:
Carpeted with the hatch for the spare:
In the car:
Since the floor doesn't move at all, the box sticks to it like glue. I've never seen it move an inch. But hopefully soon there won't be a massive box in it, just a couple of smaller ones. Hope this helps some way.
Then again, that's a lot of work to do just to stop your box from sliding around lol.