Warren, that trick is as old as the hills, but still works fine, I have done a water tank for my dad's G scale stuff that way..
The soup cans usually have a ridge at the top and the bottom that stick out from the side of the can. I wrapped masking tape around the can between the ridges until the surface of the tape is flush with the ridges. I then set the can on sheets of wood or card stock, traced the circumference of the can on to one sheet, and cut out the circle. then I would plank the card circle letting the wood extend just a tiny bit past the circle.
one the glue was dry, I glued the circle to the bottom of the can/watertower. Once that glue was dry I sanded the round edge until it was flush wit h the can I used a square to make a vertical reference line down the side of the can. then I used that reference line to start gluing boards down the side of the can, I eventually ended up with a space that was too short for the last board. I marked that last board, and carefully sanded it untill it would fit and then I glued it into place.
Note, this makes a cylindrical tank, most tanks were wade with a tapper, and were wider at the bottom. the difference is subtle, and it is very tricky to build the correct taper in, but most will never know or notice.
I used copper wire painted black to make the bands _Bands closer togerther at the bottom, and farther apart near the top.
the other card or wood circle can be used to build the base for a roof, I have done conical roofs, and octagonal roofs, and neither were just right, so I'll let you figure out your own way, and trust that it will be at least as good as my efforts.
Quite nice! the curved edges of the roof, the bands, and the band joints, as well as the supports, look like you nailed it down pretty well You did a great job with the color on the tank, and the Herald. Great work all around!