Put this on the head of your C2 flashligh. Works with C2, C2 Pro (Nichia, WR etc). Won't work with C2 Pro Max.

Add fuzzy skin to the bulge in the slicer to get better diffusion.

Transparent PETG is an optimal choice for the filament. Nylon is great, but for that you may want to scale the model about 101.5% to get good fit. PLA or hard TPU may also work. However, on high power the flashlight gets quite hot, so the diffusor may deform and become too loose.