Next we'll make the fiber optic "flag".
I used a variety of different sizes of side-glow and end-glow light pipe and fiber optic cable for my bike flag. You'll want long pieces that will dangle out of the top of your smaller 1/2" tube.
Wiedamark is a great source for fibers of varying sizes and lengths.
Even them up at the end and cut them so they're all lined up together. I added a couple rubber bands around the fibers so they fit tightly inside the tube without sliding around.
The LED will be attached to the larger outside tube which will be permanently attached to the bike. The smaller tube containing the fiber optics slides into the larger tube, and slides back out again for transport.
Use gaffer's tape or duct tape to secure the LED to the back of the microcontroller. Then, securely tape the larger tube over and around the LED.
I drilled a couple holes in my bike frame and attached two screw clamps from the hardware store to hold the tube in place at the angle I wanted. Then I zip-tied the battery pack to the frame with the on/off switch facing outwards. Make sure everything is really secure!
Slide the fiber optic tube into the outer tube and push it down all the way until the fibers catch the light from the LED and glow!
The cool thing about this setup is that you actually get some variation in colors. The 3W LED actually has three LEDs inside: a red, green, and blue. Some of the fibers will be positioned directly above one of these lights, and some will be more generally placed, so you get a lovely color variation in your fibers.