Cache,
I would try some of these insoles under your present ones if you have room. They should take up less room than felt and they will have a higher insulation value per unit thickness over felt. And they are constructed using aerogel which has the highest insulation factor of any known substance.
http://workingperson.com/products/22...soles_TFM.html
Here's an interesting article to read about the aerogel material.
http://www.designnews.com/article/13...ls_Heat_Up.php