Chomley-Warner wrote:(perhaps not cotton - linen maybe,...)
Use un-waxed linen string - strong and virtually rot-proof.
I've a large old roll of natural/undyed (light khaki coloured) linen which I use to stitch 37patt canvas repairs (perfect colour match!) and whip the ends of WW2 Brit tent guy-ropes with (looks just like the original stuff). It looks just the ticket on my ID tags too.
(P.S all those that are wanting British ID tags; "Tim's Tags" appears to be the best place to get them made - the material all looks and feel correct and he stamps them up in the proper style)