How can I fuse galvanized sheet metal together so that it is water tight?
You can make watertight trays from flat sheet metal without soldering or riviting.

Take a sheet of metal whose


length is the length of the finished tray plus two time the height of the sides;
width is the width of the finished tray plus two time the height of the sides.


Fold the sheet so it ends up looking like this:



You can fold up one side at a time using a piece of wood (maybe 1x2), a bit shorter than the length of the side you are working on, as a bending edge.

Work around the piece, clockwise or counterclockwise. 

You will have to work the angle fold in the corners as you move to each successive side. The angle corners will point outward initially. Then you bend them inward toward the short end.

All edges will end up at the height of the sides with no seams. The easiest kind of waterproof.