I assume that you'd like a fairly simple rectangular domain? With idealized forcing, perhaps? I suggest that you look through the example domains at NOAA-GFDL/MOM6-examples to see if there's something similar to what you want. Adding islands is a matter of changing the land mask input file if you are reading an external grid. Otherwise, you'd have to edit the land mask in the user source file for your application. Really, we need more information about exactly what you want to do.