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I used Nichrome in place of resistors using Al's sheet.
1333mm of 0.33mm Nichrome from Jaycar came in at 18.4R for my Canon 200mm prime lens. I double checked with an online Nichrome calculator. Commercial heaters typically use 0.75 watts per inch.
At 12v the heater can be handled. It's as warm as a freshly brewed hot beverage in bare hands.
Operating over 2 nights ambient down to 3C it worked flawlessly using an Arduino and logic level n channel MOSFET (low rds on) for switching. At the suggestion of an IIS member, I applied a dew point calc, as the heating control reference, polling a humidity/temperature sensor - a freetronics breakout board, again from Jaycar.
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