GoPro HERO Session Snap-On Lens Protector

Real basic lens protector. Slots over the front with friction fit, enough to protect it in a crash (in this case, for quadcopters).

It fits quite loosely on mine so pretty easy to pull off but stays stuck if you throw the camera around like a crazy person. Maybe play a round with the scaling if you want a better fit.

UPDATE:

Made it a little better. The previous version was visible in the video.

UPDATE (v3):

Made it a bit rounder, hopefully stronger.