Use water pressure and a cavity to form sheetmetal parts, with either city water pressure or an inexpensive high-pressure pump. See detailed coverage of the pressures required for various sized parts, the inexpensive pumps available, and some engineering designs for the cavities. We make aluminum shapes that are smooth and free of marks, although other types of sheetmetal can certainly be "inflated" into some interesting shapes.
A large fabricating shop in the greater Boston area used to use a power steering pump to form the domed ends for pressure vessels. They would weld two large discs – up to 1/2" x 8′ dia. – together around the edges, and weld a pipe coupling in the middle of one plate. Sometimes it took a couple of hours to get the desired results, but it worked like a charm.
Leaks would sometimes pop up a few times, but they would just partially drain it and weld-up the leak and start again. They had about a 5 hp motor driving the power steering pump. The pumps would die occasionally since they weren’t designed for water, but another one was only $25 from the junkyard. I saw them doing some 3/8" stainless ones once, and the whole job was done in less than a day.