Straightening a steel sheet without a press

[Railroadman]

I removed the front sheet from my '66's utility box. Overall it's solid, but years of having heavy stuff tossed in there has left it looking like a potato chip. The finished result does not have to be perfect but there's a lot to be desired at the moment. It measures about 51" wide by 24" tall.

My thought is to support the outer edges by 6x6 timber, place a 2x6 or a 4x4 on top, and wallop it with a BFH enough times to bring it closer to desired shape. Anybody disagree or have a better idea? I do not have a press but do have a pretty hefty sledge.