Another way would be to start with a circle remove all vertices but the ones required for a corner.
Add a mirror modifier and tick the highlighted options:
Place the 3D cursor Shift-S Cursor to center In object mode set the objects center to the 3D cursor.
In vertex select mode extrude E the vertices to that the mirrored pieces connect.
In front view select all and extrude along z-axis E,Z. and scale constrained to x,y: S, Shift-Z. repeat this.
Add a subsurf modifier:
Select the sharp corner Alt-RMB and increase edge crease Shift-E.
Result after closing the top:
http://blend-exchange.giantcowfilms.com/embedImage.png?bid=795