"Locking" plug for broken autobailer?

Anyone know of a plug or stopper that fits very securely into the ~1/2" opening left by my snapped off plug in the autobailer? I'm thinking something similar to this:

...except sized to fit that smaller hole through the floor.
I would ideally like to hole-saw the entire thing out and replace it with a plastic one (saw the thread elsewhere here), but this is my first season with the boat and I am well over budget already with things I can't work around... so if I can find something reliable to solve this problem this season, I'd love to. The boat will be moored, so I need something I kind of trust to not pop out on its own. I don't know what the above image is CALLED, so it's hard to search for one. The runner up will be a rubber stopper from the hardware store, but I like the idea of something that "locks" fatter.
Thanks,
Luke

...except sized to fit that smaller hole through the floor.
I would ideally like to hole-saw the entire thing out and replace it with a plastic one (saw the thread elsewhere here), but this is my first season with the boat and I am well over budget already with things I can't work around... so if I can find something reliable to solve this problem this season, I'd love to. The boat will be moored, so I need something I kind of trust to not pop out on its own. I don't know what the above image is CALLED, so it's hard to search for one. The runner up will be a rubber stopper from the hardware store, but I like the idea of something that "locks" fatter.
Thanks,
Luke