public class MockUtil {
    private MockUtil(){}

     public static Object createMock(Type typeToMock) {
        return Test.createStub(typeToMock, Type.forName('MockUtil.' + typeToMock.getName() + 'Mock').getInstance());
    }

    //All inner classes uses the same name format <class name they are mocking>Mock
    public class ClassName1Mock implements System.StubProvider {
      public Object handleMethodCall(Object stubbedObject, String stubbedMethodName,
        Type returnType, List<Type> listOfParamTypes, List<String> listOfParamNames,
        List<Object> listOfArgs) {
          ...
        }
    }

    public class ClassName2Mock implements System.StubProvider {
      public Object handleMethodCall(Object stubbedObject, String stubbedMethodName,
        Type returnType, List<Type> listOfParamTypes, List<String> listOfParamNames,
        List<Object> listOfArgs) {
          ...
        }
    }
}