Rockshox Monarch Bottomless Ring Kit
Bottomless rings reduce the volume of the air chamber. This makes the shock more progressive, helping with bottoming out.
Contrary to what you'll find on various websites, adding rings isn't a good solution to stop diving, it just makes the shock harder, deeper in the travel. If you fit them as a way to stop diving, you will most likely never use all the available travel.
These Bottomless rings will fit:
Rock Shox Monarch
Rock Shox Monarch Plus
Rock Shox Vivid Air