Modular switch bezels are the devices that are used to keep them in the correct position. They are oftentimes round, threaded components that are screwed into place to keep the devices positioned as needed. They may come in other shapes, as well, though this term usually specifically refers to a ring-shaped part.
There are other products on the market that meet this definition, however, that are not rings. Some of them are designed to hold an entire block of switches, allowing them to be used to keep the switches on a block organized logically.