perhaps 7 way plaiting makes more sense than attempting 13 way

http://www.strawcraftsmen.co.uk/project10.php
http://www.strawcraftsmen.co.uk/pdf/...flat_plait.pdf

so for a 21 wire ribbon , a matter of threading 3 wires in each of 7 thin
temporary sleeving to do the plaiting an inch or so, and sliding forward the
sleeving, perhaps. Or perhaps just 7 small sliding rings of sleeving, and
advance them, just ahead of the plaiting.