Rigid makes an internal wrench which will grab the inside walls.
You can find one at a large plumbing supply house.
A tub in my house has the lift-and-turn type of tub stopper (ie no lever/stiker assembly) and cross arms inside the stopper body have broken completly off. Thus, the stopper came out and now I can't figure out how to get the stopper body unscrewed from the tub with nothing to grab on to to turn it.....please any ideas???? Thanks in advance.