Hi,
A year or two ago I built my own USB dew heater. It was a very narrow space where to place the heater band on the camera lens, I needed a special solution. It was aimed for my portabel equipment and it would be practical with USB 5 Volt as a power sourche.
Here is my project if you find it interesting:
http://www.astrofriend.eu/astronomy/...ting-band.html
Beware, the USB power backup battery can be very dangerous if handled in wrong way.
Lars