If you just take a piece of sheet metal (galvanized square heat run metal is good) and bend it to shape (over a 4x4 works OK, or add a piece of wood to it to make a larger form) and then fold the ends up like wrapping paper. Of course a little more strength is needed to fold the ends but with the 4x4 still inside the form and with a mallet and a wooden block or two the ends can be formed quite nicely. They will be water tight and no soldering or welding is needed. A bit rough looking perhaps but it works fine, and cheap.
