Planet Bike and Serfas make taillight/reflector combinations that work well. These cost only $15-$25. They're bright enough for most low light and nighttime use. And the transparent shells serve as diffusers too, which helps with visibility even when vehicle headlights aren't aimed at the reflector -- passive reflectors can't do that.

And these are much more friendly to fellow cyclists in group rides and on the MUP than the ultra bright Cygolite, RadBots and similar taillights that focus the intensity with lenses or reflectors to concentrate the light.
Last night a ride monitor on our nighttime riding group asked me to turn off my new Cygolite Hotshot 50. I agreed -- it's way too much for that purpose and there's no way to turn it down, other than in steady mode. The various flashing and strobing modes all peak at maximum intensity -- only the durations can be adjusted.

So both of my bikes have taillight/reflector combos. I use the brighter lights only in traffic on public roads, day and night.

Regarding mounting security, the Planet Bike Rack Blinky 5 type mount works well. It hasn't bounced out even on the roughest roads, yet it's easy to unclip and remove if necessary.

But I don't trust the similarly designed plastic clip on the Cygolite Hotshot 50 on anything other than the included mount that bolts to the seat post or elsewhere. That clip doesn't have enough tension to hold reliably.

The Blackburn plastic clips on the 2'Fer and similar lights holds much more securely. It has more tension and little nibs to grip more securely.