I've had very good results using aluminum flashing from Home Cheapo for cowl flaps. Also K&S Brass sheet but the aluminum is much cheaper and not to mention, lighter..
I forget what the original purpose was but the aluminum I got was in the gutter and roof flashing area. They were cut into squares that had been bent into a corner piece. They cost less than a dollar apiece and were formed in a right angle from a piece that was roughly 6"x 6" square if I'm making myself clear. About 1/32" thick or maybe a bit thinner. Easy to form and very stiff so it keeps it's shape. . It can be cut with a pair of good utility scissors.