Rockshox Sid Luxe Volume Spacer kit 2021
Volume Spacers 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 spacers 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.
Rockshox Sid Luxe/ Volume Spacers
These volume spacers will fit:
Rock Shox Sid Luxe Ultimate
Rock Shox Sid Luxe Select+