Big suction cup in the center. Even the ones in the shower should do. 3" or better. Best if you have the metal hot (like a sunny day hot, not roasting hotdogs hot). It helps the steel relax. Pull with suction cup and hold. A simple jig to hold it a bit past flat. Let it set a while. How long has it been pushed in? The longer you have the debt, the more it wants to stay a dent.