fiberglass box

37Z

See link below that I lifted from "chops" thread.

Clear Hook-and-Loop Superlock™ Fasteners (4-Pack) - RadioShack.com
or,
My thoughts- I would attach wires to the backside of the rear of the sub box. The wire could then be attached to something (an existing bolt) located in the sub floor area of the car where you can access it by removing the spare tire. I case you need to remove the sub box; all you would have to do would be unbolt the wire. Make sure you wrap the wire in rubber or plastic wrap where it comes in contact with metal edges to prevent the wire from cutting through. Strenghten the area where the wire attached to the sub box by adding a washer and glass over it to prevent movement and prevent the wire from pulling through if you have to stop the car suddenly. Similarly, add a washer on the wire where it bolts to the car.

Try rubber strips or thick felt attached with spray adhesive on the outside (backside) of the sub box where it comes in hard contact with the factory trim piece. Second Skin sells a material that is made for reducing rattles (I am not sure what it is called). No need to cover the entire backside of the sub box; only where the sub box rubs hard against the factory trim piece.