To my initial disappointment the back of the thing (on the opposite end from the lens) isn't open as I expected. Instead plastic was molded into a flat plastic piece with holes, looking like it should attach to a clamp of some sort. It covered at least 50% of the circular opening and made it impossible to slip over the end of any flashlight I have. Also it would block most of the light.
But I made this discovery: if you take needle-nose pliers and pry up on the clampy-bits on the back, they will sheer off on the 90deg angle between the cylinder wall and the clampy bits. This leaves it totally open from the back.
Then, it turned out to be the perfect size to slip over the end of my L2D, and might work well on many kinds of AA lights. It's a snug enough fit to not easily fall off, but can be slipped on and off easily. This makes for a nice reading light and I might also try running one on my bike to augment the L2T I use now as a headlight. I figure the diffuser will make me more visible to traffic that's not in front of me.
Five for $4.71, you can't go too far wrong - if you screw one or two up it isn't the end of the world.
Thank you for the great tip, s777. For my P3D Q5 I use the red and white diffuser tips from Fenix-store.com. And for my L0D Q4, I use caps from Carmex lip balm sticks, which give me red, magenta, and green light.