When I used to visit Japan the hotel I stayed at used bathtub drain stoppers that were brass balls with a thin rubber-like coating. The ball was slightly larger than the drain hole. There was an attachment for a ball chain. When you dropped the ball into the drain hole it sat firmly and there were no leaks. And it was easy to remove as there was a large ball to grab hold of.
I've been trying to source these stoppers for our claw foot tubs but haven't been able to find any. Does anybody know where I could get a couple of these stoppers?