Is there some sort of small clamp that would serve the purpose of making a secure connection to the ground bundle (3 bare copper wires from 12-2 NM-B cable)? I've never seen a switch grounded ...

Ground clamps are identified for specified size ground rod and conductor sizes. This category also includes irreversible crimp connectors that are installed with hydraulic crimping tools (Photos 3a, b ...

The new 4” square clamp is tin-plated and has two wire grooves ... for direct burial in Earth or concrete. This heavy duty connector is another application solution found in The Grounding Superstore.