Hmm i dont think the (relitively) low current ( and voltage (5v)) of a usb will work a heater very well... try something using a plug-in adapter and a heater element. You could try resistance wire, hair-dryer wire (pretty much resistance wire) or a specialized product you can buy. Dont forget to add a fan of blower or someting so the air is moving, or else things will burn...
if you have an idea, let us know and we can check it out.