I need to make a lid for for a waste tank that I have. It's 42" X 16" and I was wondering if I should use stainless or aluminum and what thickness or gauge.
You won't have any problem with either SS or aluminium. The aluminium is lot easier to work with if you want to drill or cut into it. I have an aluminium lid that is nearly 30 yrs old and shows no sign of deterioration. I have been using it on a SS tank for many years.