They probably use aluminized mylar, the material emergency blankets are made of. A better option might be to one of those blankets and make a boot liner out of it. The idea is to reflect the infrared heat your body generates back towards you rather than letting it escape. Gold is an even better reflector.
Copper socks have been around a long time, though that might be for their anti-fungal properties. But I had wondered about a plate of copper on top of insoles to redistribute heat.