I would try the following idea first: since you have a hole in the
centre of the divot why not use this with a suitable fine pitch HT
screw to draw the two pieces of the die together?
It certainly worked or me but that was in a little thinner material and
not threadplate.
You could always heat the area first (hot enough to blacken a piece of
pine wood when touched to the area) and then quickly draw the die
together. The heated alumunium deforms easily.
You'd have to practise that on an offcut first to see if the overall
flatness gets distorted by the process.
Klaus