Nylon? I would think this would be a ‘print the moulds we use to pour in silicone’ situation, if we’re going for usability. https://formlabs.com/blog/casting-silicone-guide/ On an engineering note I’m also sure I’ve seen this exact thing before, but I wonder what would improve it. Self-heating? DIY solution could be a hidden pocket for reusable handwarmers lol