Part of the City Base range of models I have developed for use with my current 40K project. Bases are modeled with a 5 x 2.5mm opening in the bottom to accept a 2 x 5mm magnet. Depending on your first layer you may need to trim around the opening to ensure the magnet can fit.
These bases have been successfully printed on a Ultimaker 3. You shouldn’t need to include for supports however reducing infill and top layers adds additional random texture to the surface if you want a less flat surface to prime.