I just installed a stereo in my Brother-in-laws Tracker this weekend.  His is yellow, and I suggested replacing all his interior panels with Diamond plate.  But, I would paint the Diamond plate Black (matches his yellow Tracker).  I then suggested that he could keep them unpainted, untill he's ready to have more items powder coated black.  Then he could take them all down to the powder coater at one time.

I hate the stock interior panels, well at least the fasteners, if you do not have a fastener removal tool, you will tear the carboard.  Especially if the interior is older and exposed to the weather (Junkyard finds).  Those darn plastic pieces either break where they slide into the cardboard, or refuse to come out of the holes in the metal body.  Then, they some times get to brittle to hold anything.  Kinda of funny, either they are brittle and hold nothing, or they are hard and can't remove them.  It's either one extreme or the other.

If I were to do the diamond plate, I would grill it as well and mount the speakers behind it.  If done properly, you could weld the grill to the diamond plate and weld studs on the back to hold the speakers on to the diamond plate.  (Of course ,you would need somebody with an aluminum welder unless you decide heavy steel is more your style)

Then when attaching the diamond plate to the tracker, I would use bolts/screws that have a torx head or something other than a Flattip, Phillips, or Six Point Head to help prevent theft.

Now for the funny, we were installing the last speaker, when all of sudden my brother-in-law slipped of the screw and drilled a nice hole in his new speaker. 

...anyone thought of that high density plastic they make cutting boards out of?  Light weight, strong, and different.

I like this idea, maybe with a little research you could find a plastic piece with a nice pattern or flat surface as well.