Dies for forming copper sheet

I'll start by saying that I am CHEAP. That and I like to make things.
So when I upgraded my computer I skipped buying a CPU heatsink and
made some water blocks out of copper until I got one I liked. I use
it and a Harbor Freight fountain pump ( HF 41198 ). It seems to work
well in that the temp reported by the Bios is 110 degrees F. Max for a
Athlon xp2400. The pump just sits in a stainless tank with maybe
three quarts of water.
Anyway the last water block I made using a recess in some oak that I
milled with a ball end mill. And I used a ball pein hammer to form
one piece of copper. After making that part I brazed it onto a
thicker piece of copper and brazed in a couple of copper tubes for the
tygon tubing to fit on. I used SIL-FOS WITH 15% silver ( BCuP-5 ) and
a hand held propane torch to do the brazing. I annealed the copper
about three times while doing the forming.
The recess in the oak is 40mm by 55 mm by 11 mm deep. Now my question
is if I were going to use two dies to form the copper, what would the
first die be? My thoughts are something like 20 mm by 30 mm and maybe
8 mm deep. I am thinking this would stretch the copper more in the
middle. But I don't have any books with info on forming using dies
and the local library is not likely to be much help.
Dan