These are some of the custom cores that I have used or have come across. There are so many variations possible due to the type of attacks and gems used.
If you follow the links on the right, there are some custom core pictures of what players have used. You can copy them to give you an idea what they are. I’ve got no idea what players used inside of the gears so don’t ask. lol. There are also custom core for infantry, calvary, ranged and debuff custom core.