Model created in Zbrush. It is watertight and ready for printing/milling. Although there are some slight undercuts, it was created with molding and casting in mind.
I decimated STL and a highpoly OBJ. Please note that the OBJ files are rather heavy due to polygon density, so make sure to choose the print version for Your slicer.
Nominal size of the merged medallion (although easily adjustable in any 3D software) is: