After you file the edges of the base or platform of the body, you need curve the back end of each wall.  Since the width of the base is 2.7″ then the radius for each wall is 1.35″.  Then after forming the walls the  on the back so that you can solder the back and then bend the front to match the base.  You may have to bend the front a little farther because of the “bounce back.”  After you shape all of the curves in front and back, solder one side to the base. How I was able to solder the walls to base at a near perfect degrees by using four 90 degree magnets and spaced then about 3/4 inch from the, area I was soldering two to a side.  I then cooled the metal with acetone  then I soldered the next area.  After I soldered both sides to the metal it looked like this.