if you have the MP3 sub, you can mount some straps to the floor to wrap around it - should be good enough to keep it from floating around. (at least thats what i'm using) the straps also make it easy to remove if you want to take it out.
thanks man - and, you do know that the MP3 sub and the MSP sub are completely different, right? the MP3 sub was a tube that bolted to the trunk floor, and the MSP sub was a panel that mounted up under the rear deck. the brackets for each are NOT interchangeable.