I built something similar for my race Z. I used .050” aluminum on the sides, and for the top and bottom the material is about the thickness of a beer can, about .010”. I used the lower right and left areas to funnel air for a second brake cooling duct on each side.
Originally I had a sheet of lexan across the upper half, to block off the right and left upper half, and perforated the center for the radiator duct. After a bit of vigorous bump drafting the front got messed up a bit so the lexan went away.
I really should replace it, the appearance was much better.