…a solar-powered bike light, modified from a solar flashlight. (I did find one product that is advertised as a “solar bike light”, but it’s not good: it uses an incandescent lamp rather than LEDs, and it’s a one-piece unit so the whole big thing has to mount on the handlebars). With my home-made version, the solar cell and batteries mount on the rear rack, as you see here. Link.
DIY solar bike light