If your clever with a bit of electronics, you can try an heater strap (made with nichrome wire) and an ebay item
http://www.ebay.com.au/itm/12-24V-8A...9d0b5dd&_uhb=1
There are a few people on here using them... if you have a 12volt Sealed lead acid battery it will last you ages and give off enough heat to the lens to keep the dew off...