You also mentioned that you had 4 flat sheets of metal, each 1.6 feet long. You wanted to trim one end of each of the sheets and then join them together at the sides, so that you could make them into a pipe, which is 2 feet wide. It is great that you had figured out that you could cut each of these 4 sheets the same. The problem you were facing was to find out a way to cut these sheets. I am happy to tell you that I succeeded to solve the problem.
L is the length of the flat sheet of metal measured also in feet.
The curve of this function is shown on Figure 1. So here is what you have to do. Four flat sheets of metal put together. In case you are wondering how I managed to come up with such a solution, I'll show you how amazing calculus is. Let us look at your tank and pipe from the side.