Hi Harry,
I have been building square riggers for years now and have never tried the commercially available jigs you can buy. I use a pair of medical tweesers, long nosed grips and a pair of sharp embroidery scissors. Tension can be done by applying a drop of superglue at the block end, before continuing to finish the stay or shroud.
A quick tip. Keep a block of bees wax handy and run all your rigging cord through it before use. It helps to keep stray fibres from standing up.