Had issues with my flashlight turning on in my pocket so made a set of button protectors / caps. One is solid and has to be removed to turn on the flashlight and the other has a raised rim around the button, Both cases the cap just slides over the end of the flashlight and locks in place with a partial twist. There is a part that slides under the top of the belt clip and keeps the cap from pulling off until rotated. 

I printed mine in PLA using a 0.4mm nozzle. Turn on supports "everywhere" to support the locking paw during print. Likely will print fine in any material as long as that paw has supports. 

Step files and FreeCAD 1.0.2 models included.