I took photos when I made my shrinking disc. Keep in mind I'm a furniture maker and don't have many of the tools many metal shapers have so I had to make do with my tools.
Photo 1 - shows piece of Ebay stainless .043" and cost less than $10 bucks delivered, backing pad $6.00 (Harbor Freight) and Phenolic disc $5.00 from the local auto body supply.
Photo 2 - Drawing 9" circle on two pieces of 3/4 birch ply to be used as hammer form.
Photo 3 - Tacking the two pieces of 3/4 ply together. Wood/drywall
screws could be used.
Photo 4 - Cutting the hammer form on the band saw, saber saw could be used.