This is a re-make of willie42's Casting Ring, but with an alignment notch so that I don't have to pay attention to the alignment of the two halves. There's also a chamfer for perfect centering for the eventuality that the clearance is too loose (it isn't loose in my print setup).

The outer diameter is 80mm, height 50mm, and wall thickness is 3mm. The clearance between the parts is 0.2mm.