The sheetmetal/mastic idea will work fine. If I were you, I'd find a way to stop that rust right NOW. You can use whatever your preference is - Zerorust, POR15, or that stuff they sell in the hardware stores. The point is to chemically treat it now to keep the rust in check until you have time to fix it properly. Because, as we all know, you never get back to a job as soon as we think. By that time, you might have to spend a lot more money fixing what you didn't spend time preventing....
