Back in the late 80's I sent for info on making my own vest. The fellow also told me how to adjust the regulator on a BMW R65... due to the wattage that the alternator on that machine. So I tweaked the rugulator as BMWs for the most part don't charge more than 13.2 volts.
Instructions had me find multi strand wire.. #27 which is also insulated. First time I sewed only 100' feet of the wire and it did get HOT! Took the vest over to a friend's home and watched his eyes light up due to the instant heat. He advised adding 20 feet.
Nice and warm... WARM. Especially now with a higher output alternator on the BMW K series.
Start in one pocket of a vest or jacket that is lined and sew 10 foot size wire thru, never crossing over the wire but wind it around the vest or coat and then solder the sections then add shrink tape. 2 wires hanging out the one pocket and solder the zip cord to that and tie it off inside the pocket.. then attach a power plug directly to the battery... Plug the vest into that power plug and its up to the owner to either unplug the vest or add a switch.
Works.. for about 20 bucks.. kicks out 95 watts..