Mrs. Madness - have you tried using the little "hand warmer" "feet warmer" packets from an outdoors store? We use these when we go camping/skiing, hiking in fall/winter, etc.

There are two kinds - One is small enough not to irritate movement but is a one-time use deal...and the other uses a fluid that you light and heats the unit and reusable. The difference in cost I have seen is $3 for a pack of 3 or 4 or $20 for the reusable unit.