I suppose a small threaded rod would be ideal but don't know if I've ever seen anything with a small enough diameter.
I would suggest PTFE rod...available in loads of diameters, flexible, can be cut to exact length with scissors, and can be self threaded using whatever it is you intend to thread on the end of it.
I would suggest PTFE rod...available in loads of diameters, flexible, can be cut to exact length with scissors, and can be self threaded using whatever it is you intend to thread on the end of it.