I recently completed a DIY rear light and thought I'd share. It uses two Luxeon I red-orange stars powered off 3 AA NiMH batteries. Resistors are used to limit current to the LEDs. Current is about 300 mA to each LED with fresh batteries then decreases from there. I put in a rainproof toggle switch and rainproof power connector. The housing is a waterproof enclosure with removable clear cover. I frosted the cover with spray-on Krylon frosting. It didn't come out as good as I'd like but it works.
Run time is about 5 hours. Just on/off, no flash mode. It puts my Mars 3.0 to shame, particularly in off-axis visibility. Things I'll probably modify down the road are to add a third LED and run them in series off a boost converter and possibly diffuse the light a bit more to get a larger apparent light source. I need to make a more robust mount for it too.
The lumens are about 10% higher than the attached graph shows because I increased the current after taking those measurements. I used 53 lumens/watt as calculated from the Lumileds datasheet to determine the lumens numbers. Total cost was probably $75 or so including charger. The total weight is about 300 grams.