I havent done it on that car yet, but i have been working with 'glass making kick panels,boxes etc. Just make a speaker ring, use some dowels to get the angle you want and use the dowels as "stands for the ring and glue it to the panel with some CA glue. Take some fleece and wrap it to the shape and start adding resin to the fleece and build it up with fibelglass mat and resin, heres the basic steps
Nice. I knew you would have something to look at. I actually started the process doing just what you talked about. I will post some pics when I get a chance.