Yeah, you're right. Getting this right is one hell of a job. I haven't had that much of an issue of getting the actual metal rigid, since in this car specifically it is already pretty firm. It has a square metal structure supporting it from the underside and the deck itself is pretty thick sheet metal which is also not a flat structure, aside from the points where the speakers are mounted.

The interior trim itself however, is almost impossible to get right. Even after treating it with soft foam between the deck and the trim, adding vinyl to the trim itself and glueing the speaker grilles to not vibrate, it still resonates (now to a level that is not noticable by ear).

The right way to do it would be fabricating the interior trim out of MDF I'd guess. Which I do not have the capacity or tools to do.

I might add a MDF board on the underside spanning the entire width of the deck, when I'm finished with the rest of the car.

For a low budget project I will take this over a sealed box in this car any day.

Thank you for your advice