Protective Lens cover from GoPro fits nicely

[Maxwellian]

Got a C3 mounted on the front of my motorcycle, and was a little worried about flying pebbles and so forth, found that the protective lens for a gopro is a perfect fit.

https://www.amazon.com/gp/B00F19Q3T2

 

[GJHS]

You can also slide a 25.5mm step up ring on the cable side, it fits perfect over the lens module and add a polarizing filter or clear filter.