Masonite board
a trunk port is called a trunk because it is part of the back end of a car - ie a trunk.
trunk
the back thing of a car is a boot or often know as trunk. Also on top of the trunk is a thing called a spoiler
the boot
Check under the trunk liner for the factory build sheet.
On a car, the rear compartment is called the "boot".
The boot or the trunk.
Cargo area.
The net in the trunk of your car/suv is called a cargo net and is used to keep things separated and from falling out
A woodie is a kind of car, where the rear is entirely made of wood or a material that looks similar to wood, like wood-grain sheet vinyl as the industry stopped using real wood eventually.
It is called a spoiler.
Not sure what is rattling: trunk lid, or side panels. But, a speaker system needs rigid enclosures, and that includes the car itself. or even in your home. the house and the car are part of the speakers. for instance: Wood paneling in homes will rattle from low frequency sound... Removing all cardboard or fabric paneling in the trunk, and spraying undercoating or polyurethane foam would help a lot to dampen and firm up the enclosure (the car :) ) or, put carpeting on the sides and floor, but they need to be securely attached to the sheet metal (contact cement might work). Reinstalled the panels as well.