Using a Planet Bike Super Flash (PBSF), I thread a few thick elastic bands through the back vents and under the rear of my Giro Atmos helmet. Placing the PBSF on the flattest part of the rear of the helmet, I put the ends of the elastics over the area of the light that doesn't cover the bulb. If one elastic band doesn't hold it, try strapping it down more tightly or use more than one elastic band.

The elastic bands need to be replaced every now and then, but it works.