[Pault]

Some sort of strong cable should work. Or just a bar that the catapult itself hits, although that might be harder to make structurally sound because it is higher up. Also, using the cable has the added benefit of being able to adjust when the catapult hits the hard stop fairly easily.

I would also recommend using a potentiometer to stop the catapult in code. Not only will this reduce the wear on your robot, but you could actually use it to adjust the release angle of your shot. This should only supplement a hard stop, not replace it, because I can almost guarantee that something will go wrong with the code at some point.