There are a few DIYs out there, but nothing like "This is the way you are supposed to do it to achieve optimal results". I think 4 Tennis balls is a good amount; one to each side, evenly spaced, and wedged into a tight space so they are pushing up on the deck as much as possible. There is also a 'bolt mod', but I did not really understand how it works once I actually got in the subwoofer space. They talk about it here towards the end of the thread:
https://forum.e46fanatics.com/showthread.php?t=429992