While this is a project I completed slightly before watching this course it is exactly the kind of thing you could do with this knowledge (and with the information from the course I will be able to improve upon).

The image below shows 3 stages of a simple open sandcast that I am learning blender to improve upon. The Celtic knot pattern I used here as a practice piece was taken from one of the many online resources and then edited to make it useful as a positive to make the imprint in the sand.

The 3D printed positive is the top left, the (slightly scorched) imprint in the casting sand is centre and the cast pewter is bottom right before I've attempted any finishing or polishing.

I am hoping to eventually make metal plates that I can include on my leatherwork, such as belt buckles or detail plates.