How does velcro work for the large light that you described in the
beginning of this thread? I've had problems with velcro having a bit
of wobble in many instances, which would bother me with a helmet
light.


Surface area surface area - why you need the strap formed to to the
helmet shape. For the big light, I used some heavier strength velcro
(I believe it comes in 3 shear strengths and I used the middle one for
this - McMaster-Carr if you have trouble finding it). There is a
little wobble, but not much. The real problem was getting the velcro
to stick to the helmet.