Are you after a little bit of high level awareness for the car behind the car behind you or are you after a rear light of similar power to a frame mounted one which may be rather eye searing at head level for those following you?
Awareness ones, the topeak front and rear in one unit, a knog (ono) rear, lezyne femto. More eye searing intensity: many of the AAA upright types, smart 1/2 watt style sit well on the back of a helmet.
You do need to consider the rake and vents if using the back of the helmet as the mount point, along with your riding position, if you're quite head down style you could end up doing little more than signalling to passing planes.