A Balanced idea - Promaster Lens Cap
It clips in just like others, but features a translucent white center section that allows your camera to take White Balance readings in the custom mode - and to get an accurate WB for unusual circumstances. Consult your camera manual - in the case of all my Fujifilm X cameras it is as easy as one click with this lens cap.
Saves a heap of time later when you are finishing your jpegs!
Standard sizes up to 77mm.