Just thought of another source of thin sheet aluminum!
If you have a sign shop in your vicinity, you have access to aluminum sheets in almost any size, because that is basic sign material. The only thing about sign aluminum is that it comes prepainted, usually white on one side and yellow on the other side. This could be an advantage since the factory paint coating is bonded to the aluminum far better than any workshop process is likely to produce and even if you have to paint over it, paint sticks to paint far better than it does to bare aluminum.