cut a one inch wide styrene strip using .015 inch thick styrene. Make the strip about one inch longer than the circumference so you can glue the strip onto itself while it is wrapped around the shell of the tank. You will now have a band you can scribe against to make the horizontal weld seams.
To make the vertical seams, I use an architects scale. It is shaped like a triangle and will lay on the shell so you can scribe against it.