For long focal lengths, almost any thin metal works well. I prefer the ends of tin cans. They are strong, easy to drill, and the thickness doesn't really matter when the diameter of the hole is many times the thickness. For wide angle pinholes, thinner brass shim stock might be better. Accurate and clean holes can be drilled by tightly sandwiching thin brass between fairly hard material like tempered hardboard. For such large pinholes, this technique is faster and more accurate than the usual dimple and sand method.