I don't know a source, I have only seen guys make one-off's for their projects. Most guys who cant weld use rivets to attach stuff.
To make it easier to shape the metal and not need welding you can leave open corners. Corners that aren't welded together can have a longer flap in order to shape the box flap over the side rails. As long as the battery is strapped down you won't get significant forces that would distort the tray in that case.