advice for sub rattles/placement/mounting

I've got a basslink subwoofer bungeed down in the hatch right behind the passenger seat.

I think i'm getting some low mid vibrations that are set off by the subwoofer. They may be coming from the hatch, or from the cavity underneath. I do have various things in the pan under the tire. So the questions are, is there a better location, or preferred mounting that can help. If not is there deadening material that I should be putting in the hatch or underneath on the plastic piece over the spare.

Thanks
Ed

 

I think I may fill the tray with towels and t-shirts and see if the vibrations go away. If it does, I wonder if there is some sort of thin liner I can put down. I'll also check that the tools under the tray are secure and rattle free. I really like the tray (friends who had seen how disorganized my trunk was in the previous car, get surprised that everything looks put away in the prius.) I'm hoping its the tray or things under it. The cover is also a suspect maybe I'll pull it out as a check.

I do have some tools in the tray, but I can wrap them. I'm lucky here that I don't need a snow shovel. It doesn't snow here often, and if it does no one expects you to drive (too many people on the roads that will slide into you).