I would stay as far away as possible from Aluminum unless you can find some exotic alloy that won't corrode. As mentioned above Acid in any of the many products that can be dumped down a drain will cause problems, think urine, citric acid etc. I know you like to make things your self and many times that is the best way to get an exact fit. If you need the space filled to the max,I would go fiberglass then you have something to fit the space and a custom job as well. Just watch the resin you use as some types are more water proof than the others.
