Put a cheap thin plastic sandwich bag over any light, pull it tight over the lens part of the light. Bunch it up on the back and put a elastic around it.
Even better is pull the bag tight, cut off any excess, carefully fold it over, and tape it with scotch tape. If you do it carefully and this is not a light that gets taken off and put back on a lot, the bag will last a long time. This works on headlights, bike computers, and cell phones. I always bag my cell phone. I pull the bag tight so the front of the phone has as few wrinkles as possible, the helps to see the buttons easily, and then fold and tape the back with any kind of tape so the back is pretty flat. You can talk and listen right through the bag. It's great to keep the phone dry in a sweaty jersey pocket.
If the bag gets ripped just replace it. The bags are almost free and weigh almost nothing.