Slightly bigger than the end of your thumb and weighing 14g, this blinky LED is tiny.
Its elastic strap loops around a seatpost, seatstay or helmet vent and fastens - in seconds - over a couple of plastic fins, needing no bracket. It stays put well, although a rubberised underside would be even better (I used a sliver of old innertube).
The single LED (steady or flashing) is very bright, albeit not legal for use by itself at night. It's a good backup, though. I used it flashing on my TT bike on gloomier evenings to alert dozy motorists. Uses two CR-2302 batteries (included).