C10 audio component install behind seats

[TurboSixPiston]

That looks like a cover made to fit over a fiberglass/mdf amprack and enclosure. Metal of any kind is not an ideal material for building an enclosure for a speaker. Covers, racks, supports, that's all fine, but not the actual enclosure. Better left to fiberglass and wood.

This can be made with fiberglass by first cutting and bending wood into the shape you want, attaching a dense and stretchy cloth over it conforming to the shape you want, then applying a coat of resin. Allow the resin to cure, add 4-5 layers of fiberglass mat over the entire surface, smooth it out with your chosen technique, and finish it. Because the enclosure sits behind it, it does not need to be the strongest creation on earth, simply beefy enough to move around and secure into place without distorting.

Benefits of using fiberglass and wood over aluminum or other metal:

Cost.
Basic skills required, easily done by a beginner who can plan and construct wooden frames, then follow the directions given by countless websites and magazines for laying fiberglass.
Minimum equipment required being a jigsaw, nailgun/screws and drill, a worktable, a couple clamps, latex/nitrile gloves for easy cleanup, a filtered mask, plus materials.
Cost.
Cost.

Benefits of using aluminim:

It has that wow factor of shaped metal that's hard to duplicate with any other substrate.
Not nearly as messy.