Best designs for bike lights are 1) small, 2) 90%+ eff, and 3) reliable. There are several ways to do this. Here are a few i have done.
LM8040 micro module http://cds.linear.com/docs/Datasheet/8040fa.pdf
This is easy, just wire up and package it.
Zetex 310 A buck hyst type controller:
I did this with surfboards and hand wire. Very small, fairly simple, 85%+ eff.
The brown wire power in, follwed by a 0402 fuse and zener for over voltage protection.
L1 is off board, as is L2 . The order of componets is swapped in this schematic, F1 should be to the right of L2.
R1 is the little green blob in the upper right. most componet values here are "suggestions" and not exact.