Here's an odd solution, but...
Do you happen to know anyone in the priniting industry? The reason I ask is because a friend of mine built a fish house and sided it with discarded aluminum printing plates. I don't think you can just ask a printing company for old plates because they probably recycle them for $$$. But if you know anyone that works in a printing plant they might be able to hook you up. Probably not the most aesthetic solution but it would cost you next to nothing and it's effective. My friend worked in the plant so he got the OK from his manger to take some. Like I say, you'd probably need a connection but it worked for him!
"My friend asked me if I wanted a frozen banana and I said no but that I was going to want a regular banana later so... yeah."
R.I.P Mitch Hedburg (1968 - 2005)