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It really is easy, Richard.
Use Constantan resistance wire. Measure out a length that, when connected to 12v, warms to the appropriate temperature. Thread these wires (probably four or five lengths will be enough - two sleeves, two on front/kidneys, one on back/shoulders/neck) through the lining of lightweight jacket/shirt, etc., and grip the ends together (they can't be soldered). Do the same with the other ends and connect to a switch and then the battery! Easy peasy!!
I can't even sew or knit !
