Sounds like a good idea. My regular LED flashlight is plenty bright, but it wipes out my night vision. I've got a piece of red lighting gel somewhere around here, I can't believe I didn't think of using that already. Thanks.
/johnny
I took this photo off the internet to use as an example, different size bottles or drilled caps (I like sliding the light backwards into the hole, and then screwing on the cap) for different size flashlights, or one can just lay the flashlight inside of an oversize container, or one can buy fitted diffusers for brand name lights (or for measured sizes of flashlight heads, usually in mm).