Brass Cantled Sporrans - Need Historical Guidance

[artificer]

I would love to find out how the metal for the cantle is formed into the semi circle with three sides, i.e. a front surface, a back surface and a top.

In most cases, the front of the cantle is only two visible sides; the front face and semi-circular top. The back 'face' is a separate piece that hinges down at the bottom where the lowest knob is (assuming there is a knob there, some lack them).