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

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: