Nick Odell wrote:

I sometimes use knitting needles as a cheap source of plastic or
aluminium rod for markers along fretted instrument fingerboards. Would
something like that work for you?

It could well, thanks for the thought. It's also occurred to me that
round-headed nails could maybe be cut down, painted and glued into place.
The advantage of ready-made white plastic plugs would really just be
convenience and speed, plus perhaps the neatness of a slightly domed head.

Bert