Wahoo? This is the base for a CycleAware Reflex mirror. Fits mine just fine.
That said, as designed, it only makes contact with the helmet at a tangent point...helmet is curved, of course. So I thinned it down a little, designed and printed a mating piece that's flat on one side, curved on the other, glued them together. Will stick that on my Botrager WaveCel in the next day or so, but expect it to be solid.
-Richard