Marshall Electronics CV503WP-CAPS Replacement Lens Cap

The CV504-WP goes outside, which is exactly why its front cap gets knocked off a mount, dropped down a truss, or left behind in a case. Marshall sells the cap on its own as CV503WP-CAPS: a black threaded barrel that screws down over the camera's lens housing and closes the front of the optical path when the camera is stored or in transit.

One caveat on fitment. The cap is cut for the standard-length prime lenses in the WP family. Marshall's longer M12 primes stand proud of it and take the extended CV502WP-CAPS-LG cap instead, so check which lens is fitted before ordering.