There's a whole internet
of geometric info on the stellated dodecahedron. For now, you need to know that it's made of identical triangles which have a height : base ratio of 1.54 : 1. For this particular mold, my triangles are about 3-5/16" on the base, 5-3/32" in height. Like people, they don't have to be perfect.
Cut yourself a pattern, tessellate them out on a strip of steel, and slice them up with your method of choice. I used the hydraulic shear at my friendly neighborhood makerspace, but jump shear and plasma cutter are fine alternatives.
In total you'll need 50 triangles, but having extra is always nice. For this size mold that comes to about 3 sq ft of steel, and I chose a healthy-looking crop of 16-gauge.