I was at Walmart today. In the craft section by the faberic, that had plywood silhouette cutouts of apples, dolls, and other stuff. I thought about buying some and making firewalls out of them. They were $0.79 each.
I have made a firewall with a table saw by cutting a thin sheet of oak with the grain, and cutting out the final shape on a scroll saw. It was very strong. I think I will just order some next time.
For control horns, I use gift cards, per Experimental Airlines.