I don't know if others have used this technique, but I have made a couple of spinner rings and needed to open out the sides with a spherical tool. Not having any punches big enough I did the following.
Put my brass coming block in a jug of boiling water and press the mouldable plastic in the dome size you need, then as the brass transmits heat very well you get the dome quickly.
Run both under cold water and voila one dome punch. As long as the metal is annealed then the punch usually works and can be melted again to another size.
I bought the plastic at Mindsets online, they provide scientific products to schools and you can find amazing stuff there
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