Kat81 one thing you might try to keep your reader from freezing is to try using some of those hand warmer pads in your back pack. If you put them in a nonflamable sock or something of that sort you will be able to keep the contents of your backpack warm even in subzero weather. There are some that work off of batteries or air activated. Some of the products at either one of these sites might be of use for you. Or you can use one of the many available search engines to help find something more suitable for your needs. Hope this advice helps.
http://www.campingsurvival.com/handw...FSkZawodezPv0A
http://www.safetycentral.com/inreushanbod.html