Been testing out the Planet Bike Flipper, a seat post mounted fender, for a couple months this winter. In a strange occurrence, I gave the Planet Bike Blinky 7 a run for it's money in the wet.
You can see how the light was mounted on one of the rack braze-ons on the seat stay... which placed it in a prime location for all the spill from the fender. ...and the light never stopped working, ever... for months. I can say this light is one of the most durable products I have ever used.
Now back to the fender...
With two knuckles to pivot on, the mount gives you everything you need to place it over the tire to keep all the muck and such off. It has a quick release for the seat post that is adjustable, but it wraps around and clamps on allowing it to move if bumped. You can tell too, as you go from dry to wet pretty quickly. Once I added a small piece of inner tube between the clamp and post for added friction, no big deal and issue solved.
So how did the fender work? The tool roll under the saddle stayed dry in snow, rain, downpour, and drizzle. When I took the pic, I had just finished an hour or so in driving rain with zero issues. That is what you want, to put it on and not have to worry about it.
If you are looking for a clip on fender to keep your ass dry, or your bike does not have braze-ons, the Flipper is perfect. ...and it's only $20.