There are other threads here that deal with diffusers ("candle mode" tends to mean taking the head off of a mini-mag, putting the tail of the light inside the head, and then using the bare bulb/LED as a candle -- some of the terms used on this forum do not translate as well as other terms).
Some small pill or vitamin bottles fit snuggly over the top of the flashlight head and make great diffusers (white plastic containers tend to work best). A little bit of a waterproof tape would solve your problem. However, you will not get full brightness from the flashlight with the diffuser in place.
I've said before we could use a thread of internal diameters of various containers and outer diameters of various lights so we'd know which fit to what.