I think aluminium would work fine for what you want to do. What you might do is get some small peices of aluminium like 2"x2" and make test peices. Paint them with diff paints and such, you can even get that rhino liner stuff in a can. I don't think it's "Rhino liner" but the same kind of stuff? Put these peices in a hot part of the eng, but where it will cause no damage like next to the exhuast manifold to see what will stand up to the heat. You might even try some of that BBQ Grill paint, they make a few diff colors for grill paint. If it was me I would just go with the aluminium look or maybe sand it to give it that brushed look. Aluminium will get hot, but it also should cool down fast too. When or if you try the peices try differant thicknesses to see what will get hotter and stays cooler. You don't have to put it next to the manifold it was just an idea, but where you are putting the shild at would be just fine.
Good luck, I will like to see the finished product.