Students at MIT have developed a number of example geometries that can be used as templates. They are now public as Jupiter notebooks on their GitHub repo: GitHub - mit-crpg/openmc-reactor-examples: Catalogs a plethora of reactor inputs for OpenMC
Keep in mind that these geometries are based on generic designs and may not exactly fit the geometry you seek to create (see the sources listed in each notebook). Please use these geometries as templates to be modified for your specific use.
We hope these help you save time!