A rear lens cap for an M42 screw mount lens fits perfectly to cover the side grip unit mounting threads when the side grip is removed. Helpful to know if your thumb rest is missing or lost.
Replacement lens caps are readily available online. Just search for "rear lens cap M42 screw mount lens"
Just purchased a set of two (2) that also included the matching camera body cover. The body cover will probably work along with a 20mm extension tube to protect the grip unit's connection when not attached to the camera.