Now to show the formation of the dimples. Once again, use what you have available, but here is a good use for old bed frame angles. Cut to size, weld together, and weld another strip around the top edge. This will allow more surface area as we radius the inside to prevent cuts in the metal. Don't forget a strip of metal across the open side to keep it from spreading..
The top edge gets trimmed down to size, our hammering will go easier without all that excess in the way.
I have a couple different sized "anvil punches" that I have formed radius ends on, we'll use the large one initially, and a smaller to fine tune the shape afterwards.
With the dimple formation, we are actually shrinking the overall length of the part in this area, as can be seen by the distortion in the top flange:
Some of this will be removed when we bend the offset in the flange, but in the end, it will need some stretching in both areas after the forming is complete, in order to restore the proper length to the part.