Bontrager Flare 3 rear light mounted on the seatpost.
The Moon Comet Rear Light does not fit the bill as the mounting bracket causes the light to stick out quite a bit. The Cateye Rapid X does blend in nicely with the seatpost, but it is rather expensive at over $50. After looking around, I found a rear light by Topeak that can blend in nicely with the seatpost.
Topeak RedLite Aero Rear Light
The "Aero" in the product name does not just refer to how aerodynamic the rear light is. It also means that this rear light can be mounted on aero seatposts that are not cylindrical in shape. The aerodynamics of a rear light does not really make a difference as it is so far back from the front of the bike. However, what attracted me to it is that when mounted, it blends in nicely with the seatpost.
2 lighting modes, Constant and Blinking.
Comes with 2 rubber straps to suit different sized seatposts.
The light grey rubber strap is more stretchy and fits larger seatposts.
Closer look at the rear light
The yellow button at the bottom of the light is the On/Off button and also changes the lighting mode between constant and blinking.
Note that the LEDs are tilted at an angle. This ensures that the light beam is directed more or less horizontally when mounted on the tilted seatpost.
Uses 1 x AAA battery. This is covered by a rubber back cover.
Weighs 33 grams which is rather lightweight
Specifications of this rear light. A nice long runtime of 80 hours in flashing mode.
Seen as mounted on the seatpost. Although the rubber back cover does not fit tightly to the red casing, the rubber strap will pull and seal the back cover tightly when mounted on the seatpost.
Blends in quite seamlessly with the seatpost, without any clamps or mounts protruding from the seatpost.
Decent side visibility from the 3 bright LEDs
Very aerodynamic look when viewed from the rear of the bike.
How it looks when it is turned on
Bright enough to function as a primary rear light on its own
Comparing it to a Smart 0.5watt rear light. It does not have such a bright focused beam, but does have a good vertical coverage. Horizontal coverage is not so good though.
1) Decent side visibility
2) Blends in well with the seatpost
4) Slim and compact size
5) Easy to install and remove from the bike with the rubber mount
6) Excellent battery life
7) Rain proof
1) Can only be mounted on the seatpost
2) Beam is not as bright as other lights with more focused beams
As you can see, there are many good features for this rear light, and very few negative points. Most importantly, it blends in nicely with the seatpost which helps to keep the nice aerodynamic look of the road bike.