Magna bend using a piece of round stock.... You could accomplish similar by clamping a pipe across your flat stock on the edge of work bench and bending the sheet around the pipe... then just make corners and weld them in separate.
Alternatively, you could make a completely new bottom, turn up the 4 sides with the radius, and tuck shrink the corners to keep it all one piece.. You don't have a lot of height that is needed, so the corners would not be that hard to form over a rounded post anvil. Here is a video of a similar operation, although a much smaller scale. The theory is the same, however...
http://s5.photobucket.com/user/rmcca...eo158.mp4.html
Please excuse my lack of camera location awareness, my first feature film...
