I used a piece of old aluminum sign, about 1/8" thick...sanded it almost to the point of being able to polish it, but went ahead and gave it a brushed look with red scrotchbrite instead. Then I gave it two coats of lacquer clear to seal it.
I had planned on a full polish but it had a few minor flaws.
I liked the brushed look anyway so I ran with it instead.
Used some poster board to make a good fitting template and transfered the shape over to the aluminum. Cut it with a jigsaw just a little outside the mark and sanded it to size to better control the final shape.
Stainless screws hold it to the wooden firewall.
View attachment 13904