I took a 3/4" thick wood board and cut to amplifier size. I figured out exactly where I wanted it to go on the floor and semi secured the board under the carpet with some tape. Then, push the carpet back on top of the board and screw the amplifier in over the carpet through to the board. The amplifier will tighten over the carpeting and firmly secure its position under the seat.
To make this easier I also cut away some of the foam which was under that portion of the carpet.