This may be what you're looking for:
Temperature Sensing Duct Heaters
* Completely programmable duct sensing electric duct heater.
* Automatically senses air temperature within ductwork
* Regulates electric heat output based on predetermined desired temperature.
* Flexible temperature ranges from 40 – 100 degrees
EM-WX0111R 1 kW/3414 Btu/h 120V plug 6” Round Element
150 cfm
http://www.electromn.com/res/html/duct.htm
see pictures and assembly:
http://documents.northwrite.com/Shelter/ductheaters.pdf
disclaimer: I haven't tried those and don't know anyone who did...