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I pulled the insole out of one of my boot so I could take some measurements of my DIY heating pad (which replaced a non-DIY one that degraded over time). Since its clear you can see how the 2 parallel heating loops are arranged. I arranged the carbon fiber on a piece of polyethylene vapor barrier, holding it in place with pieces of kapton tape. Then put another piece of poly on top, sandwiched with aluminum foil and bonded them using a clothes iron for some DIY lamination.
It helps to have the pad wider than the insole so wraps around outside of the foot. You can buy pads like that on ebay / aliexpress for way cheaper than you could make them. Most are 5V USB and probably don't get very warm, but you could run them at 9V to make them warmer.
I'm still using the original 7.4V batteries that came with the original pads, but am looking to make a pair for 9V USB-C. Also going to try using carbon tape instead of rope. Might just use heavy duty ziploc bags to laminate instead of the thicker vapor barrier.
