Hi,
i did build in a spiral feather from a Bosch Lighting Dynamo with the specs Ø14mm- x 5 x 24,5mm, but i would recommendent a slightly longer arm, that means the specs Ø15,4mm x 30mm x 6mm.
Costs 2,31€.
And it is possible to give these worn out and old feathers a rather better tension by heating them up (with a heat gun), increase the tension with a gripper and colling or quenching them with water or oil. I tried it by support of an expert of treating metal - and i was impressed by the result. It works!
Hilsen, Sascha