To get that metal bent to a square corner clamp a 2x4 or other standard size wood or an angle iron where you want the corner and bend it at that point and make the corner square with a mallet or other handy hammer. If you want it round, clamp a piece of pipe where you want the bend and bend it and use your handy hammer to make it a uniform bend. Obviously clamp it to both edges so your "break" doesn't move till you're done with it.It would be best to have a piece of wood or angle on both sides of what you're bending but one will work as long as it's on the inside of the corner. If you don't have clamps screw or bolt the pieces together, but you can never have too many clamps for woodwork or metalwork.

Thanks for the tip! That old tin sure does come in handy --- Mark keeps finding new ways to put it to use.