The best system I have found to keep my flanges warm are brain and smoke tanned deer skin and Norwegian knull-bended wool liners – hands down! Northern indigenous cultures use a process of smoke tanning which keeps the mittens durable and supple. It also increases the leathers insulation value and makes it more breathable.
The key to the wool liner is that its natural lanolin is preserved, which helps keep the liner drier and allows me to beat the frost out of it at the end of each day. An added bonus of this system is that the smell of wood smoke reminds me of home.